I understand you not wanting to hurt her, but please realise that you won't be doing that. You'll be telling her about your true self, nothing more nothing less. How she reacts to that is down to her, if she chooses to feel hurt by you being trans* that is her choice.
However, something else to keep in mind is she might also be upset that you didn't tell her sooner, that you kept a "secret" from her. She might want to know lots of answers, some questions will be weird, rude and/or personal...she may want to know answers to things you don't have answers for. She'll probably see it as thing that effects not just you and her and your relationship but also her sexual identity as well as her friends and family. It might take her a fair bit of time to process it all, she might need space too.
In case you're wondering, yes I've been in a similar situation - even though it was eight years after I'd detransitioned and wasn't planning (at that stage) to retransition she didn't take it very well. And I had thought she was a very open minded person! Not so as it turned out. But yes, she had felt I'd kept it a secret from her (we'd been together about five months before I told her), she wondered if I was really gay, worried about what her friends would think if they ever found out, blah, blah, blah.
Thing is you can't really know until you tell her, and even then it may take some time for it to be fully resolved. Do you have a sense of what your girlfriend thinks about gender in general and trans gender people in particular? Might be good to get an idea of that before launching into a full blown revelation.
All the best, but remember you are not hurting her.