I think it’s really important to get her parents’ feedback. As much as the event is about her, it’s also quite possible that it also important to the family - perhaps in a way that would not fully welcome you.
The fact that you would be there not for your own child’s celebration but their daughter’s could make your attendance feel graver in a way. Or you could be crashing a rare family gathering prompted by the graduation.
Otoh, if they want to share the event with others to make it more celebratory, you would increase the happiness quotient.
The graduation is to celebrate the graduate, but in fact, it’s a rite of passage that is also important to the family. This one marks a special shift in life staus – end of student-hood and launch to real life, a permanent address that isn’t yours, etc. If the GF parents want to do this without you, there will be another that carries less emotion and sentiment.
I really like @cinnamon1212 's advice. It gives everyone room to say what’d be best for them and to think the best of each other.