In this case "G" would stand for "Get". "F" represents the term "Fucking", and "R" is for the word "Real". G.F.R can be useful as an expression in situations where one would encounter the unbelievable or fantastic. Also used as a request to get serious about a certain discussion or matter at hand.
G.F.R. Is that cop really pulling us over?
G.F.R. You spent how much of your trust fund on lotto scratchers?
G.F.R. they throw in a book light if you order that blanket with sleeves bro.
G.F.R. dude, your girlfriend never got a flu shot? Tell her she cant come over...