1) Means "Go Fuck Yourself".
Commonly used yiddish
2)It can also be taken literally.
It then means "go jerk off".
Mark: Hey babe, have fun last night?
Rachel: HAHA mark,you can Gai tren zich now.
Mark: Guess not.
Slang for stoner or heavy user of pot.
Cate: I'm going out with ricky tonight.
Jess: Um, why do you like him? he's a complete pik.