In the not-too-distant past, Gap has used Anja Rubik, Sasha Pivovarova, Liu Wen, Sean O'Pry, etc., so it's not as though they never spend money on more in-demand models.
That said, they're not the kind of brand the NEEDS of-the-moment models in their campaigns. They're known for jeans and wearable, affordable casual clothing for the middle class. I worked at Gap when I was in college and I was the only employee at the smallish store I worked at who even knew who Anja Rubik was y name. I'm sure most of their target demographic doesn't know the difference between a good model who wears the clothes well and a very-famous-in-the-fashion-world good model who wears the clothes well. Adam Driver is probably a better use of their money than the people that a more exclusive brand might use. I don't think it's them being cheap, I think it's them being smart.