Job Title: Graduate Research Assistant
I would suggest that you not claim any title that wasn't assigned by others. You can give a description of what you did or suggest that the work was "similar to GRA" but that is different from saying that you actually had the title. Your only official "title" was student, I think. 

While what you say might be accurate in some sense, the problem is that someone seeing official documentation and not finding you listed with a title you claim, all start to wonder why and even thing you have given a fraudulent answer. While that isn't your intent, you don't want to open the possibility of such an accusation. 

I suspect that in Germany it is especially important to be accurate about titles, though that may be changing. 

But, for a CV, have a section on experience with a very brief explanation and a phrase "similar to ..." that gives the person the right idea about your duties, but not claiming a title. 

You could also, in a mail or email, ask the institute what you can call yourself on your CV. But I'd suggest that you want the answer in writing and from someone official if it isn't a formal position you held. Then there can be no question of your intent.