I've set a forwarding address, i receive e-mails sent to gandi mai on my gmail account. Everything is fine so far. But when i want to send an e-mail from gmail using my gandi mail name (so that receiver think that i've sent the mail from my gandi mail, no gmail) it doesn't give me such option. I tried configuring something in gmail settings/accounts importing but i couldn't manage. Can anyone please help?

Hi Rihards, I have the same setup as you. You need to manually register each forwarding email address into your Gmail settings, like this:
1_ From your Gmail account, go to Settings / Account and Imports tab.
2_ In the section "Send mail as", click the button "Send mail from another address". This triggers a pop-up.
3_ Fill in all information (your name, your emailalias@gandidomain.tld, etc), and then let GMail send the confirmation email.
4_ After a while, you receive a confirmation email into your Gmail inbox (provided Gandi redirection worked well).
5_ Click the confirmation link from the mail.
6_ Now whenever you send or reply a mail, you can select this Gandi alias from the "From" listbox. Note that GMail is smart enough to set the good alias by default when you reply to a Gandi-forwarded email.