Green’s Dictionary of Slang

(orig. naut.) a glutton, a heavy eater; a greedy person (for items other than food).

1929		F.C. Bowen Sea Sl. 56: Gannet, a greedy seaman.
1943		Post (Lanarks.) 7 Feb. 18/4: Services Slang [...] Navy [...] Gannet — greedy seaman.
1950		P. Tempest Lag’s Lex. 91: gannet, any glutton [...] The bet may be on how many plates of porridge one ‘gannet’ can put away at a sitting.
1962		H. Livings Nil Carborundum (1963) Act III: Look at that rotten gannet.
1987		Beano Comic Library No. 121 51: Mama Mia! What a pair of gannets!
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 76/1: gannet n. 1a selfish, greedy person.