Of course you're in the right place! ^.^ This is where we come to help each other over these kinds of questions! ^^ Alright...here's my thoughts...
Ok, so, your girlfriend kind of knows, kind of doesn't...? It might be best to try to bring her onto the same page. The family is a very tough one everyone has trouble with, but I personally tried to establish it with the parent I trusted to be rational and open minded about it. Either way, be careful that revealing will be on your own terms, (don't let them find something and freak out D: ) and be confident. If they seem to not be taking it well, things that might be worth explaining are 'I'm still me, always will be', and 'It's not some mistake in growing up, (or 'being with you' as relevant) it's just that I was born this way and I'm finally coming to terms with it'. (it's helpful to read into what the brain/body bit's about to understand it better) I had a third one in mind, but forgot it. -.-;
Anyway, at the same time, don't try to bash them over the head with it if they're not comfortable with it, because this is something pretty big to take in. Don't expect them to be entirely ignorant of transgenders just because they're cisgenders: I noticed a transexual friend of mine does that, and I don't think it's right that she discredits groups like that. :\
Finally, this is a big thing for you, not entirely them. It is something that impacts the people you know, but the person this ultimately matters to is you. It's good to establish people you know will stand behind you, but it's also important to not lag yourself excessively simply on the account to getting everyone with you.
I hope that helps ^^