Hey, welcome aboard!
I love giving people advice and I think you can get some good advice here but you should be taking advantage of your built-in expert - you GF. If she is like most vegans she will like being asked about veganism, and she will love talking about it. And she should surely appreciate your efforts to please her.
The other thing that will soon become readily apparent, there are many different types of vegans. And what she likes and wants to eat will depend a lot on what kind of vegan she is.
BTW, checking the allergy listings on a box is a good idea but may not be necessary. And to some extent, this will depend on what "kind" of vegan she is. For instance, I pretty much ignore the "may contain" warnings. IMHO, these are CYA statements made by the company just in case someone dies from eating their product. The "may contain" warning doesn't mean the eggs or milk or whatever was put in on purpose but that there may have been some contamination or that there may be an accident.
These "may contain" warnings are usually because the facility also processes milk or eggs and there IS a possibility of cross-contamination. I choose to ignore those. However, if the label leaves off the word "may" and simply states that it "contains milk" or "contains eggs" then that IS a nonvegan product.
Anyway, as i said, I choose to ignore those, and I think most vegans do. but there are vegans who ARE concerned with cross-contamination.
Those allergy warnings are a good place to start but not where you can end. Reading ingredients is a skill almost all vegans had to work on developing. There are also some details that can trip you up. One of the details that trip up vegans is that carmine, cochineal extract and natural red 4 are food colorings made from bugs.
There is an app called IsItVegan? that is pretty good and if I'm in doubt and I have the time I'll check the product in question with that. but mostly I just go with the idea that if I'm in doubt I will just use something else.
The thing that most gave me trouble is bakery items. Most of them are made with butter, or eggs, or milk.
Another detail is white cane sugar. But only some white cane sugar is not vegan. Personally, i eat so little of the stuff that I decided not to worry about it.
Ha. Now that I have given you all this advice - well, you should check it out with your GF. She might be very concerned with the "details" or like me not that worried about them.
OK, last piece of advice. Check out a vegan cookbook or vegan website and share some recipe ideas with your GF. There are probably vegan foods she doesn't like. For instance, I can't tolerate spicy foods so I don't like chili. but many vegans have chili on the top of their lists.