Gandi offers a webmail interface through Roundcube. You can use this to log in to any of the emails you have set up through Gandi.
The login for this email will be the email address you set up in your account. The password will be the password you assigned when you set up the email.
You login with the email address and password you set up with Gandi, not with your Gandi login and password.
You can find information on how to use this interface in Roundcube’s documentation.