Green’s Dictionary of Slang

(Aus. prison) an indeterminate sentence; a person who is serving an indeterminate sentence owing to insanity, usu. for murder.

1984		B. Ellem Doing Time app. C 207: ‘He bears watching that bloke, he’s likely to go off any tick of the clock, he’s doing GP and you can’t trust him’ [...] Any GP that’s got an ounce of intelligence will conform and do the right thing, he won’t buck the system.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 G.P. Acronym for Governor’s Pleasure ie a person serving an indeterminate sentence by reason of insanity usually for murder. Used by prisoners in a pejorative way because of the implication of madness.