GM Force Weapon

For the GM force weapon it states that all normal psychic rules apply so I'm assuming it's possible to be affected by Perils of the Warp. My question is if the GM wounds a model is the psychic test optional or is it mandatory? Can he elect to just keep it as a normal power weapon attack? Just want to be clear on it.

You don't have to make the Psychic test to kill the enemy model, but you might as well try to against multi wound models, but there's no point in making the test for single wound models (they're already dead) so you don't have to.

No, he can just use it as a S 6 power weapon attack.

Is it also just one psychic attack per combat phase even if more than one wound was originally inflicted?

You can only use one psychic power per turn, so if you have already used a psychic power you cant use the force weapon ability. You can only use it once per assault phase, and not if you've alredy used a psychic power, and you can't use another psychic power afterwards. (Unless you have consecrated scrolls or whatever) The codex says that no mattter how many wounds are inflicted, you can only make one psychic test, so to finaly answer your question, yes.