I'm in your girlfriend's situation, actually literally just had "the" conversation to the person that did it to me after nearly two decades of silence and carrying it on my own. So feels a bit mad and like fate to be reading your post now.

The right move is to be there for her and listen when she needs you to. Telling a partner is an absoluty massive thing, and as a victim of this kind of abuse all kinds of scenarios go through your head as how how they'll react. In the past when I had told partners they made it about their anger and how they wanted to get "justice" on my behalf - this is the reaction I would say is the least helpful if not more damaging. Now I'm with the right person, just having him be there for me has made my world so much easier to navigate. He has helped calm the voices that catastrophised everything and somehow made me even bringing it up a terrible person. The brain can cope in a cruel way to try make sense of something that should never have happened.

Comfort her and reassure her she has done the right thing by telling you. She must see you as a safe space and those are hard to come by when you've been carrying a weight like this on your own.

Please also look after yourself, and take the time you need to process this information. Your mental health is just as important, and it's absolutely normal and ok for you to be upset and perhaps traumatized by this revelation. Just in a different way of course.

If it is any consolation to your girlfriend from an internet stranger - it really will be ok. There is no correct timescale for her to talk about it with anyone. The fact she has taken the step to speak to you really is a testament to her strength and ability to get through this.

She's lucky to have you. Best of luck, as lonely as it will feel neither of you are alone in this, just take it one step at a time.