gull (n.1) Look up gull at Dictionary.com
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) Look up gull at Dictionary.com
cant term for "dupe, sucker, credulous person," 1590s, of uncertain origin. Perhaps from verb meaning "to dupe, cheat" (1540s), earlier "to swallow" (1520s), ultimately from gull "throat, gullet" (early 15c.); see gullet. Or it is perhaps from (or influenced by) the bird (see gull (n.1)); in either case with a sense of "someone who will swallow anything thrown at him." Another possibility is Middle English dialectal gull "newly hatched bird" (late 14c.), which is perhaps from Old Norse golr "yellow," from the hue of its down.