gull (n.1)

shore bird, early 15c. (in a cook book), probably from Brythonic Celtic; compare Welsh gwylan "gull," Cornish guilan, Breton goelann; all from Old Celtic *voilenno-. Replaced Old English mæw (see mew (n.1)).

gull (n.2)

cant term for "dupe, sucker, credulous person," 1590s, of uncertain origin. Perhaps from verb meaning "to dupe, cheat" (see gull (v.)). Or it is perhaps from (or influenced by) the bird name (see gull (n.1)); in either case with a sense of "someone who will swallow anything thrown at him." Another possibility is Middle English gull, goll "newly hatched bird" (late 14c.), which is perhaps from Old Norse golr "yellow," from the hue of its down.

gull (v.)

"to dupe, cheat, mislead by deception," 1540s, earlier "to swallow" (1520s), ultimately from gull "throat, gullet" (early 15c.); see gullet. Related: Gulled; gulling.

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Definitions of gull from WordNet
gull (v.)
make a fool or dupe of;
Synonyms: fool / befool
gull (v.)
fool or hoax;
Synonyms: dupe / slang / befool / cod / fool / put on / take in / put one over / put one across
gull (n.)
a person who is gullible and easy to take advantage of;
Synonyms: chump / fool / mark / patsy / fall guy / sucker / soft touch / mug
gull (n.)
mostly white aquatic bird having long pointed wings and short legs;
Synonyms: seagull / sea gull