hello, how do i forward a gandi mail to another mail address?

It's right on the setup page for your domain. Just go there, click the Email section. You have to first activate Gandi email, and then you can set it to forward specified email addresses to other places.

That works for "forwarding accounts", but you cannot send email from such account.

Technical support has confirmed real mailboxes cannot be forwarded. This is a pity, that feature is handy.