Online Etymology Dictionary

-gamous: word-forming element meaning "marrying," from Greek gamos "marriage" (see gamete) + -ous.
-gamy: word-forming element meaning "marriage; fertilization," from Greek -gamia, from gamos "marriage" (see gamete).
-gate: suffix attached to any word to indicate "scandal involving," 1973, abstracted from Watergate, the Washington, D.C., building complex, home of the National Headquarters of the Democratic Party when it was burglarized June 17, 1972, by operatives later found to be working for the staff and re-election campaign of U.S. President Richard Nixon.
-gen: word-forming element technically meaning "something produced," but mainly, in modern use, "thing that produces or causes," from French -gène (18c.), from Greek -genes "born of, produced by," related to genos "birth" (see genus). Originally in late 18th century chemistry and probably reflecting misunderstanding of -genes, as though it meant "that which produces."
-genic: word-forming element meaning "producing;" see -gen + -ic.
-genous: word-forming element meaning "generating, producing, yielding;" see -gen + -ous.
-geny: word-forming element meaning "genesis, origin, production," from French -génie, from Greek -geneia, from genes "born, produced" (see genus).
-gram: suffix from telegram (1852), first abstracted 1979 (in Gorillagram, a proprietary name in U.S.), and put to wide use in forming new words, such as stripagram (1981). The construction violates Greek grammar, as an adverb could not properly form part of a compound noun.
-graphy: word-forming element meaning "process of writing or recording" or "a writing, recording, or description," from French or German -graphie, from Greek -graphia "description of," from graphein "write, express by written characters," earlier "to draw, represent by lines drawn," originally "to scrape, scratch" (on clay tablets with a stylus), from PIE root *gerbh- "to scratch, carve" (see carve). In modern use, especially in forming names of descriptive sciences.
G: for history of the letter's development, see C. As a movie rating in the U.S., 1966, standing for general. Standing for gravity in physics since 1785.
g spot (n.): also g-spot, 1981, short for Gräfenberg spot, named for German gynecologist Ernst Gräfenberg (1881-1957), who described it in 1950.
G-man (n.): "FBI agent," 1930, shortening of government man; used earlier in an Irish context (1917), but the abbreviation is perhaps the same one.
G-string (n.): 1878, geestring, "loincloth worn by American Indian," originally the string that holds it up, etymology unknown. The spelling with G (1882) is perhaps from influence of violin string tuned to a G (in this sense G string is first recorded 1831), the lowest and heaviest of the violin strings. First used of women's attire 1936, with reference to strip-teasers.
I AM the spirit of the silver "G":
I am silvered sadness,
I am moonlit gladness,
I am that fine madness
Of reverence half, and half of ecstasy
[from "Spirit of the 'G' String," Alfred L. Donaldson, in "Songs of My Violin," 1901]
G.I.: also GI, 1936 as an adjective meaning "U.S. Army equipment," American English, apparently an abbreviation of Government Issue, and applied to anything associated with servicemen. Transferred sense of "U.S. Army soldier" arose during World War II (first recorded 1943), apparently from the jocular notion that the men themselves were manufactured by the government.

An earlier G.I. (1908) was an abbreviation of galvanized iron, especially in G.I. can, a type of metal trash can; the term was picked up by U.S. soldiers in World War I as slang for a similar-looking type of German artillery shells. But it is highly unlikely that this G.I. came to mean "soldier." No two sources seem to agree on the entire etymology, but none backs the widespread notion that it stands for *General Infantry. GI Joe “any U.S. soldier” attested from 1942 (date in OED is a typo).
gab (v.): "to reproach," c.1200, via Scottish and northern England dialect, from a Scandinavian source, e.g. Old Norse gabba "to mock," or from Old French gabber "mock, boast," both perhaps ultimately imitative. Related: Gabbed; gabbing. Meaning "to talk much" is from 1786, probably a back-formation from gabble.
gab (n.): early 14c., "mockery," from Old French gab, from gaber (see gab (v.)); meaning "idle talk" is from 1737. Gift of the gab "talent for speaking" is from 1680s.
gabardine (n.): 1590s, "dress, covering," variant of gaberdine. Meaning "closely woven cloth" is from 1904.
gabble (n.): c.1600, from gabble (v.).
gabble (v.): 1570s, frequentative of gab (q.v.), or else imitative. Related: Gabbled; gabbling.
gabbro (n.): type of igneous rock, 1837, from Italian (Tuscan) gabbro, from Latin glaber "bare, smooth, bald" (see glad).
gabby (adj.): "garrulous, talkative," 1710, originally Scottish, from gab (n.) + -y (2). Related: Gabbiness.
Gabe: masc. proper name, usually short for Gabriel.
gaberdine (n.): "long, loose outer garment," 1510s, from Spanish gabardina, from Middle French galverdine, which is perhaps from Middle High German wallevart "pilgrimage" (German Wallfahrt) in the sense of "pilgrim's cloak" (from Old High German wallon "to roam;" see gallant (adj.) + faran "to go, travel;" see fare (v.)). The Spanish form perhaps influenced by gabán "overcoat" and tabardina "coarse coat."
gabfest (n.): "session of conversation," 1895, American English colloquial, from gab + -fest.
gable (n.): mid-14c., from Old French gable "facade, front, gable," from Old Norse gafl "gable, gable-end" (in north of England, the word is probably directly from Norse), probably from Proto-Germanic *gablaz "top of a pitched roof" (cf. Middle Dutch ghevel, Dutch gevel, Old High German gibil, German Geibel, Gothic gibla "gable"), from PIE *ghebhel.

Cognates seem to be words meaning both "fork" (cf. Old English gafol, geafel, Old Saxon gafala, Dutch gaffel, Old High German gabala "pitchfork," German Gabel "fork;" Old Irish gabul "forked twig") and "head" (cf. Old High German gibilla, Old Saxon gibillia "skull").
Possibly the primitive meaning of the words may have been 'top', 'vertex'; this may have given rise to the sense of 'gable', and this latter to the sense of 'fork', a gable being originally formed by two pieces of timber crossed at the top supporting the end of the roof-tree." [OED]
Related: Gabled; gables.
Gabriel: masc. proper name, also name of an Old Testament angel, from Hebrew Gabhri el, literally "man of God," from gebher "man" + El "God." First element is from base of verb gabhar "was strong" (cf. Arabic jabr "strong, young man;" jabbar "tyrant").
gad (v.): "to rove about," mid-15c., perhaps a back-formation from Middle English gadeling (Old English gædeling) "kinsman, fellow, companion in arms," but which had a deteriorated sense of "rogue, vagabond" by c.1300 (it also had a meaning "wandering," but this is attested only from 16c.); or else it should be associated with gad (n.) "a goad for driving cattle." Related: Gadding.
gad (n.): "goad, metal rod," early 13c., from Old Norse gaddr "spike, nail," from Proto-Germanic *gadaz "pointed stick" (see yard (n.2)).
gadabout (n.): 1837, from earlier noun phrase gadder about (1560s); see gad (v.) + about (adv.).
gadfly (n.): 1620s, "fly which bites cattle," probably from gad (n.) "goad, metal rod," here in the sense of "stinger;" but the sense is entangled with gad (v.) and another early meaning of gadfly was "someone who likes to go about, often stopping here and there" (1610s). Sense of "one who irritates another" is from 1640s (equivalent of Latin oestrus).
gadget (n.): 1886, gadjet (but said to date back to 1850s), sailors' slang word for any small mechanical thing or part of a ship for which they lacked, or forgot, a name; perhaps from French gâchette "catchpiece of a mechanism" (15c.), diminutive of gâche "staple of a lock."
gadolinium: metallic element, named 1886 by J.C. Marginac in honor of Johan Gadolin (1760–1852), Finnish minerologist and chemist, who in 1794 first began investigation of the earth which eventually yielded the element and several others.
gadzooks: 1690s, from some exclamation, possibly God's hooks (nails of the cross) or even God's hocks. Cf. godsookers (1670s). The use of Gad for God (cf. egad) is first attested 1590s. Among other similar phraseological combinations (all from 17c.) were gadsbobs, gadslid, and gadsniggers; in all of which the second elements are sometimes said to be mere fanciful syllables.
Gaea: see Gaia.
Gael (n.): 1810, from Scottish Gaelic Gaidheal "member of the Gaelic race," corresponding to Old Irish Goidhel (cf. Latin Gallus). The native name in both Ireland and Scotland, Gael was first used in English exclusively of Scottish Highlanders.
Gaelic: 1774 (adj.); 1775 (n.), earlier Gathelik (1590s), from Gael (Scottish Gaidheal; see Gael) + -ic.
gaff (n.1): "iron hook," c.1300, gaffe, from Old French gaffe "boat hook" (see gaffe). Specifically of the hook on a fishing spear from 1650s.
gaff (n.2): "loud, rude talk," 1825, from Scottish dialect, perhaps a survival of Old English gafspræc "blasphemous or ribald speech," or from gaff (n.1), and cf. gaffe.
gaff (n.3): "cheap music hall or theater; place of amusement for the lowest classes," 1850s, British slang, earlier "a fair" (1753), of unknown origin.
gaffe (n.): "blunder," 1909, perhaps from French gaffe "clumsy remark," originally "boat hook," from Middle French gaffe (15c.), from Old Provençal gaf, probably from a Germanic source, from Proto-Germanic *gafa. Sense connection is obscure; the gaff was used to land big fish. Or it may derive from British slang gaff "to cheat, trick" (1893); or gaff "criticism" (1896), from Scottish dialect sense of "loud, rude talk" (see gaff (n.2)).
gaffer (n.): 1580s, "elderly rustic," apparently a contraction of godfather (cf. gammer); originally "old man," it was applied from 1841 to foremen and supervisors, which sense carried over 20c. to "electrician in charge of lighting on a film set."
gag (v.): mid-15c., "to choke, strangle," possibly imitative or influenced by Old Norse gaghals "with head thrown back." The sense of "stop a person's mouth" is first attested c.1500. Related: Gagged; gagging.
gag (n.1): "joke," 1863, probably related to theatrical sense of "matter interpolated in a written piece by the actor" (1847); or from the sense "made-up story" (1805); or from slang verbal sense of "to deceive, take in with talk" (1777), all perhaps on notion of "stuff, fill" (see gag (v.)).
gag (n.2): "act of gagging," 1550s, from gag (v.); figurative use from 1620s.
gaga (adj.): "crazy, silly," 1920, probably from French gaga "senile, foolish," probably imitative of meaningless babbling.
gage (n.): "pledge," c.1300, from Old French gage "pledge (of battle), security, guarantee" (11c.), from Frankish *wadja-, from Proto-Germanic *wadi- (see wed). Italian gaggio, Spanish and Portuguese gage are French loan-words. The verb is late 15c., from French gager. Related: Gaged, gaging.
gage (v.): see gauge. "The spelling variants gauge and gage have existed since the first recorded uses in Middle English, though in American English gage is found exclusively in technical uses" [Barnhart]. Related: Gaged; gaging.
gaggle (n.): late 15c., gagyll, with reference to both geese and women. Barnhart says possibly from Old Norse gagl "small goose, gosling, bird;" OED calls it "one of the many artificial terms invented in the 15th c. as distinctive collectives referring to particular animals or classes of persons." Possibly of imitative origin (cf. Dutch gagelen "to chatter;" Middle English gaggle "to cackle," used of geese, attested from late 14c.).
Gaia: Earth as a goddess, from Greek Gaia, spouse of Uranus, mother of the Titans, personification of gaia "earth," as opposed to heaven, "land," as opposed to sea, "a land, country, soil," a collateral form of ge (Dorian ga) "earth," of unknown origin, perhaps pre-Indo-European. The Roman equivalent goddess of the earth was Tellus (see tellurian), sometimes used in English poetically or rhetorically for "Earth personified" or "the Earth as a planet."
gaiety (n.): 1630s, from French gaieté, from gai "gay" (see gay). In the 1890s, especially with reference to a London theater of that name, and the kind of musical shows and dancing girls found there.