Every new Gloat site is set up on a subdomain like yoursite.ongloat.com. Subdomains are free and can be used forever.
You can use a custom domain for no extra cost. You can specify your custom domain at installation, or switch from an ongloat.com domain later on.
To move from a Gloat subdomain to your custom domain, follow these instructions to get your domain and Ghost site connected.
Fill in this form to leave a domain change request.
You will be sent an IP address to point your domain to.
Every domain provider has a slightly different interface, but the idea is the same for each.
You will need to create or update an A record for your domain and point it to your site's IP address.