If you have a HG reseller account, you probably use the "websitewelcome.com" nameservers, or have your own private ones.
Perhaps I was a bit too short in my previous post. You do need to first create an end user account. As a reseller you have access to WHM (which you use, basically, to create end user accounts) and cPanel. As an end user, your customer will only have access to cPanel. Then you can setup the email accounts that he needs, or he can even create them himself once you give him his cPanel username and password.
But, yes, that should be it. Hostgator should be able to help you out, should you run into any issues.