Probably from gatter, old British slang for "beer".
gattered (comparative more gattered, superlative most gattered)
- (UK, slang) Drunk.
2000 October 3, Thermostat [username], “Re: sabu424 looking for an efed”, in rec.sport.pro-wrestling.fantasy[1] (Usenet):People would have to be completely gattered to think anything nice about you.
2001 July 23, Steve Robertson, “Re: I got a really bad fucking haircut”, in alt.music.mansun[2] (Usenet):spilling sangria on white trousers when you are gattered is bad tho'.
2004 February 20, Dave Herbal, “Re: West Brom”, in alt.sports.soccer.sunderland[3] (Usenet):PS Do they put meths in the beer up there? We always arrive in the vaults about 10.30 absolutely gattered :-p