What happens if the pilot declines my booking paid for with a gift card?

You don't lose a thing. If the pilot declines the request, the balance committed from your gift card is returned to you in full.

A request with no reply

The same applies if your request expires because the pilot didn't respond: the balance isn't used up and goes back onto your card.

Your card stays usable

Once returned, the card becomes active again with its balance. You can use it again on another flight in its country of issue.