SPF
To authorize Gandimail (Gandi.net's mail service) to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanism used by them is shown below.
include:_mailcust.gandi.net
For more information on SPF please click on the button below.
DKIM
Please see below the article on how to configure DKIM on their platform
Create a free OnDMARC account to test your configuration.