I don't know if that's possible. As someone who does fundraising (including a raffle and auction) for an arts non profit, I would either select a specific item to donate, or instead of a traditional gift certificate you make a certificate that entitles the bearer to the item of their choosing (up to a certain dollar amount) from your etsy shop. You could then have the winner contact you to select his or her item. Thus providing you with their contact info for mailing the item and then deactivate the etsy listing. This would only cost you the $0.20 + shipping associated with the listing.
This may be a bit more effort on your part, but in the long run a bit easier than figuring out how to work a gift certificate.
Make sure you get a letter from the org thanking you for the item and including the declared value for tax purposes!
