Can I buy a goodybag with my credit?
You sure can. Assuming you've got enough credit for your chosen goodybag the option, “pay with balance” will appear when you're ready to buy.
In the marvellous giffgaff app, just change your payment method to "credit balance", assuming it's not already selected.
One thing to remember: you'll need to have the full cost of the goodybag in your balance. You can’t part pay with credit.