I wouldn't be upset. I would just tell her firmly that that is not what was agreed and, as Grams2five says, your DGD will have to find some other way of learning to drive.
If your DD chooses to spoil her daughter, that is one thing. You do not have to indulge your DGD just because your daughter does.
I assume your DD or DGD will be paying the car insurance. Car insurance for new young learner drivers is very expensive.
Sooner or later your DGD is going to have to learn that it is a cold hard world out there, where her every wish will not be granted and she has to earn the money to pay for her pleasures. That lesson is more easliy learnt, if the learning starts at home.