Green’s Dictionary of Slang

1. (US black) general principles.

1969

‘Iceberg Slim’ Mama Black Widow 208: I think I’ll waste you on GP.

2. (UK juv.) grand passion.

1959

I. & P. Opie Lore and Lang. of Schoolchildren (1977) 349: ‘G.P.’ (Grand Passion) is usually reserved for hero-sweethearts, such as film stars [...] thus they say: ‘Elvis Presley is my G.P.’.

3. (US campus) good plan.

1981

Eble Campus Sl. Fall 3: GP – good plan.