G-credits that come from refunds do not expire. But, there are some G-credits that might expire after a certain time. This is the case for G-credits that are awarded as part of credits back promotions, contests, or special events. If you participate in a promotion and get G-credits, the small print on the promotion will let you know the specific date in which those expire.
Once the expiration date passes, if you have not used those G-credits, you will no longer be able to use or claim them. So be sure to use them as soon as you get them!
If you have any expiring G-credits in your account, whenever you buy, the system will automatically deduct those credits first. We want you to use them! There is nothing different in the process of applying credits when those G-credits have expiration dates.
Please refer to this article for the necessary steps for applying credits to a purchase.