If you attempt to renew your domain, and you receive the error that you need to see your Reseller, then you will need to contact them with regards to renewing your domain, as they are the technical intermediary between you and Gandi. They are likely your webhost as well.
Two of our largest resellers are MoonFruit and Web Eden.
You can contact MoonFruit here.
You can contact Web Eden at the following email address: info (at) webeden.co.uk
If you have trouble contacting either of them, or you are with another one of our resellers and do not know how to contact them, please send in a Support Ticket.