I don't think that gull, in the sense of cheat, is slang at all. I would say that it's an old word, not used so much today as it was centuries ago.
126.96.36.199 14:53, 31 May 2005 (UTC)
According to Online Etymology Dictionary
c.1430 (in a cook book), probably from Brythonic Celtic, cf. Welsh gwylan "gull," Cornish guilan, Breton goelann; all from O.Celt. *voilenno-. Replaced O.E. mæw.
Jimp 08:05, 31 August 2009 (UTC)