Green’s Dictionary of Slang

gup n.2

[? gup n.1 or SE gulp]

(Aus.) a fool, a simpleton.

[Aus]Baker Popular Dict. Aus. Sl.
[US]D.X. Manners ‘Fifty-Grand Funeral’ in Ten Detective Aces Dec. [Internet] Those two bullets Dutch the Gup put into your left lung.
[Aus]Baker Aus. Lang. 130: Fools of one kind and another [...] gammy, gazob, gimp and gup.