Hi there,
If you like, you can keep your domain registered here at WordPress.com. Then, you can point your domain over to Hostgator. To accomplish this, you will need to get the domain settings from Hostgator and then apply them to your domain.
We have a guide for this here:
http://en.support.wordpress.com/domains/change-name-servers/
Alternatively, you can transfer your domain over to Hostgator by following this guide:
http://en.support.wordpress.com/domains/domain-management/#transfer-to-another-registrar
If you have questions about either process, please let me know.
Finally, I noticed that you only received a partial refund for your bundle upgrade, so I have processed the remainder of that for you. You should see those funds appear on your statement within one week.