@htc97 As stated by others you can't top up your data to your existing goodybag and it's not likely that will happen. To be fair giffgaff have some excellent value goodybags that have a generous amount of data. You should be able to find one that suits your needs here https://giffgaff.com/sim-only-plans
Looking at the forthcoming changes to goodybags even the 500mb gigabag is going although the new £5 goodybag has the same 500mb plus calls and texts so, you're probably as well to buy another goodybag early if needed. You may know that there used to be a 1gb gigabag too but the all inclusive goodybags are just as good value.