Green’s Dictionary of Slang

gum n.5

[reverse sp. of mug n.1 (2a)]

(Aus.) an extreme form of fool.

[Aus]J. McNeill Old Familiar Juice (1973) 101: bulla: You’re a dead-set ‘gum’ if ever there was one [...] Know what a ‘gum’ is, Stanley? [...] A ‘gum’, little mate, is a backward mug!!