GH GH GFU is an acronym for Get high, get high, get fucked up. It's taken from the song G.
F.U - Performed by Wiz Khalifa.
GH GH GFU
literally means to party, get drunk/high and get fucked up (smashed/wasted).
GH GH GFU can be used as an adjective or a chant, aka:
Adj: Lets
get GH GH GFU!
Chant: GH GH GFU! GH GH GFU!