I don't want to drive my girlfriend to class all the time
Why don't you want to drive your girlfriend to class?
Gas is too expensive
I am sure that your girlfriend does favors for you and it is best to reciprocate and also do favors for her. If you are not available when your girlfriend asks you to be, she will stop doing favors for you.
No, she doesn't do me favors anywhere near as often I do for her, and she never offers to pay her fair share for things.
Would you feel differently about taking her to class if she paid for the gas? Perhaps you should talk things over with her, and see if you can come to an agreement that works for you both.