Since what the mother has to say regarding you and your relationship to her ADHD daughter makes little sense, I would assume the girlfriend no longer wants to see you. That is not what you want to hear, but it would appear to be true. It is sad to lose someone you care for.
Probably, the best thing that you can do is to learn about ADHD. This site gives you tons of info - http://www.help4adhd.org/en/about/what
I like your idea about gradually getting to know the child.
I am a little worried that you have not seen your girlfriend for four months. Kind of makes me wonder if she is not using her daughter to give you another kind of message?
Oh, one other suggestion. Order the book, "The ADD/ ADHD Answer book," by Susan Ashley from Amazon (about 10 bucks). And then give it to your girlfriend. Its probably the book I recommend the most and it will really help her and her daughter (and just might help you get back into the picture :)) Best wishes.
If you have any specific questions about ADHD later on - please feel free to post!