This will depend on a few small factors
If they have an Apple or Google subscription, they’ll need to cancel it first. Then they can redeem the gift card, and the extension will be added to their subscription, and be linked to Stripe
If they already have a Stripe subscription, the gift card can only be applied if the billing period matches the gift card (3 months, 6 months, or 1 year). If it does not match and you have already bought the gift card, reach out to support@join.cc