Ross well done on thinking how to positively react to this news and reach out to an unfamiliar forum. It's always the case that of course no one should be congratulated for doing something as basic as helping another (if you can, in any way you can). But a lot of people find rape a bit of an inconvenience, truth be told. A lot of posters here who have experienced rape might have found the same thing? Difficulties in finding support from people you thought were very reliable for that.
I was raped several years ago, and have only told three people I thought would care/help (?). Their reactions varied massively. Telling people about it is so difficult, really the worst to present what's happened to others who seems so happy and fine. And they might react all sort of ways.
From this I take two things.... it's a very big deal that your gf gave you this information. Massive deal. Think how many people she has shared this with. This means she trusts you and maybe is looking for you to just recognise the fact and help. Years later I never know if I told those three people for help or just recognition of my pain, because honestly I don't know what they actually could have done to help. Maybe just listened, given their time. Like your gf the chances of prosecution were zilch and in a traumatic way I'd already processed the rape as much as possible on my own (not the best route at all!!!). So in a way I was looking just for...maybe recognition? It felt taboo to tell anyone and so Victorian, and I was disappointed massively and now feel incredibly awkward about 2/3 of the responses. So she's taken a big risk in telling you at all and might feel the need to downplay the whole thing at the moment. She might also know that as gently as you're reacting to the news, you will have a reaction (good or bad for her). So she's maybe anticipating dealing with that now, it's a burden and a big risk for her.
Her experience and her processing it are obviously massive and more than any of us can comprehend, no experiences are alike. She'll be waiting to see how you approach the subject.
I know some of the posters on here take very different tones - but that's a good thing. Your gf will have times when she feels really defensive about what she told you ("just forget it, it's nothing", or "you wouldn't understand"). She might want to push you away emotionally at times, or really appeal for you to help her make sense of it. The posters' reactions here show a fair amount of how she might feel about it in any one day. Equally she might go months without ever thinking about the rape, and certainly what someone else inflicted on her...it should be for her to decide how that shapes her identity (if she wants to identify as a survivor or forget about the whole thing).
I know people are being particular with language, but thats a great thing!! We need to make sure you are very precise with how you convey your feelings to her. People sadly do victim blame and it's not always as big as "they had it coming", but if she thinks you even possibly consider her hanging with the wrong crowd had anything to do with it...she might bolt emotionally. She won't want to have to justify her circumstances around the rape. Be very very careful around that.
You are absolutely of course going to have a reaction to this news... but don't talk over her, fumble what you want to say, or accidentally stumble into victim blaming ("why didn't you just..." etc). I used to hate it when my ex said "I'd never do anything like that to you"...like well done! Ugh. Show her you're way above that and a constant and reliable comfort. She will know that about you in other ways if she's even told you this news, but show her you can be there for this too. If she's reached out to you at least give her a clear response and think here with us about what you're going to say to her when she's comfortable speaking about the issue again.
In terms of anger - two different things. You need to rant and rave and probably want to kill the guy that did this. Do!! (Rant and rave at least). Process here as much as you can for anger management, and think of different ways of dealing too (exercise or whatnot). BUT don't take that to her. Make sure she knows you're so angry at her attacker (not the fact it happened, not her fault remember!) but don't put any emotional stuff on her about it. Don't make it so she's then supporting you about this, that'd be ridiculous.
It's awful if anything bad happens to someone you love, and to know they've been going it alone. I hope we can help you here