Panel > Email > Manage Email
Gmail accounts are listed right down the bottom and link to the setup page for each individual domain. It's fairly straight forward to run through the setup process and basically involves creating an admin account for the domain.
If Google is insistent that the domain is already registered and disallows access to be re-registered, you'll need to contact Google and inform them that you are the domain owner and that there is a problem. They'll fix you up.
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