Should you share information about narcissism with your narcissistic girlfriend?

No, it really won't help, it will only offend her and possibly hurt her feelings, though she may not show it, at least not in a productive/healthy way. Narcissistic people, (depending on the type of narcissism and the degree) usually have much deeper, more complex issues. Maybe you should see a therapist together, or seperately, (she will eventually need to see one alone, to work on her own issues). This must be suggested delicately, never blame or insinuate in any way that she has a problem, counseling is beneficial to everyone, and every relationship. You want her to be happier, right? You want to have a better relationship. This is the way to start.