I find it creepy that my gf best friend doesn't like to share almost anything about her life
That feels odd. Why do you think it's creepy though?
Because I think this shows unhealthy narcissism.
While it maybe odd that she doesn't open up, I don't think that it is a fair assumption that she's narcissist as narcissistic people tend to have more problematic attitude than what you are describing about her. If you can, try to invite her for dinner to see if she would open up to you, but if not, I would just leave it alone and respect it.
I'd feel more comfortable with some more interhuman transparency from her.
I would be comfortable with that too, but it is her life and as long as she's not harming anyone, it is her right to be as closed as possible to anyone around her.