Old English gielpan (West Saxon), gelpan (Anglian) "to boast, exult," from Proto-Germanic *gel- (source also of Old Saxon galpon, Old Norse gjalpa "to yelp," Old Norse gjalp "boasting," Old High German gelph "outcry"), from PIE root *ghel- (1) "to call, cry out." Meaning "utter a quick, sharp, bark or cry" is 1550s, probably from the noun. Related: Yelped; yelping.

early 13c., from an unrecorded Old English word or else from Old Norse gapa "to open the mouth wide, gape" (see gap (n.)). Related: Gaped; gaping. As a noun, "act of opening the mouth," from 1530s.

"stare stupidly," 1785, American English, of uncertain origin. Perhaps [Watkins] from gaw, a survival from Middle English gowen "to stare" (c. 1200), from a Scandinavian source akin to Old Norse ga "to heed," from Proto-Germanic *gawon, from PIE *ghow-e- "to honor, revere, worship" (see favor (n.)); and altered perhaps by gawk hand (see gawky). Liberman finds this untenable and writes that its history is entangled with that of gowk "cuckoo," which is from Scandinavian, but it need not be from that word, either. Nor is French gauche (itself probably from Germanic) considered a likely source. "It is possibly another independent imitative formation with the structure g-k" (compare geek). From 1867 as a noun. Related: Gawked; gawking.

Proto-Indo-European root meaning "to yawn, gape, be wide open." 

It forms all or part of: chaos; chasm; dehiscence; gap; gasp; gawp; hiatus; yawn.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit vijihite "to gape, be ajar;" Greek khainein, Latin hiare "to yawn, gape;" Old Church Slavonic zinoti "to open (one's mouth);" Russian razinut', Serbo-Croatian zinuti, Lithuanian žioju, žioti, Czech zivati "to yawn;" Old English ginian, gionian "open the mouth wide, yawn, gape," Old Norse gina "to yawn," Dutch geeuwen, Old High German ginen "to be wide open," German gähnen "to yawn."