Gandi does not force you to renew your domain names by default.
Consequently, (with the exception of .AT) if you want to delete your domain name, you just need to not renew your domain!
If you do not renew your domain, it will be deleted according to the natural deletion schedule of its extension. For a .com domain, this is 80 days after its expiration date, after which it becomes available for purchase on a first-come, first-served basis on the open market.
If you need to delete your domain name before its natural deletion date (i.e. for legal or emergency purposes), please send the domain name deletion form (and all the required proof of ID as listed on the instructions on the form) to Gandi by fax, email, or postal mail.