Technically,it is very illegal for you to handle,target practice with or use in self defense your GFs pistol unless you have a CPL,even while living in the same address.However,in reality,a jury would never find you guilty of any crime for defending yourself with her pistol in your own home even if technically you broke the law.Now long guns are totally different.If she had a shotgun,you could borrow it and possess it and so could she.and take it to the range and whatnot.