What should I bring to my GF's Family Thanksgiving?

Haha! I met my girlfriend's family for the first time on Thanksgiving, too! We were married for over twenty years.
1) Bring your most relaxed and friendly attitude, and best manners.
2) Bring a dessert from a real bakery...it will get lost in the pile. If someone likes it, you brought it! If they hate it, shrug your shoulders.
3) Bring mother personal flowers. Your girl will judge how you will treat her by how you treat Mom.
4) You're doomed. Hahaha!

It's best to ask your girlfriend. Some families have more of a pot luck, and she'll be able to find out from her family what kind of food is most needed.

Other hosts labor over a well-coordinated menu and streamline their dinner-- they may not like it when someone brings something double, or something that doesn't compliment the menu, etc.

If not a dish you could always bring flowers (in a vase-- don't let them run around looking for one and setting it up) or a plant, a package of fine chocolates, or, if you're of legal age, a bottle of wine.

Ask her mother what would be the most appropriate and most appreciated.

bring a pie and some flowers

Ask. I once took a pea salad only to be asked to leave it in the car because it had peanuts.

You could always ask for a suggestion.

Bring pie. Theres never enough pie.

Bring beer, lots and lots of beer!

Make some vegetables, or a salad. Have fun.

Go to a good store and ask for advice about a nice decoration item, perhaps for the table centre or a gift hamper of goodies, that should put you in the good books.

Chocolate cream pie
or pecan pie with a can of real whipped cream
and a bag of hazelnut coffee-coffee is always appreciated if it's good

Bring self-nudes to show his nieces and nephews.

Wings and chicken fingers for the games on TV. Her dad will think you are a champ.