How do I get along with gf's divorced parents?

Divorce. Be respectful to both parties without taking sides, make up your own mind as to your feelings about them based on your interactions. Best.
See below... Lovingly and respectfully... Toward both... Try not to take sides... Most importantly, be supportive and attentive to where your gf's at... Good luck... If you're feeling emotionally overwhelmed by their dynamics, or if personal issues are getting triggered for you, do not hesitate to get help for yourself...Take care of yourself...
Follow her lead. I don't mean do everything she does--especially if she has a hard relationship with one or both. But for example, if she wants to see one, and wants you to join her, then do. If she does not, then don't push her. You may form opinions about one or the other that differ from hers--she will need the space to work out her own solutions. If she asks for counsel, offer it gently. Above all: support her.