Since you said you would give her gas money, you should keep your word. Don't mention it again until the end of the date. When she drops you off, mention it casually. "Lemme give ya a few bucks for gas.". Then hand her $10 or whatever.
If you live in California or Connecticut, give her a fifty lol. Just kidding.
If she refuses to take the money, don't argue with her. At least you made the offer.
I don't consider it an AFC move, but I am concerned about you setting a precedent and having to pay each time she picks you up. If it comes to that, maybe offer every once in a while. Paying her every time is overkill because a 20 to 30 minute drive is no big deal.