Use of Gandi's nameservers is free for all domains registered or transferred to Gandi. In order to use Gandi's DNS, however, your domain must be at Gandi. Gandi does not allow DNS zone files for domains not registered with us.
You can use DNS servers from other service providers (also called external DNS). This is common when you set up web hosting at a hosting provider.They will often allow you to simply point the domain to their servers, which in turn direct traffic to your site.