WHMCS is a third-party software written by WHMCS Ltd. and is supported directly by WHMCS. HostGator has an arrangement with WHMCS Ltd. to provide you a free license with a reseller package or Linux dedicated server.
If you are having trouble requesting your license, please contact HostGator for assistance. HostGator agents can also assist HostGator customers with some basic issues with WHMCS (mostly limited to the topics covered in the HostGator knowledge base). Because WHMCS is not a HostGator product, some agents may have more knowledge of WHMCS than others.
For other issues, like feature requests, bug reports, WHMCS addons, etc., it would be best to contact WHMCS directly, since they are the author of the software. Also, if you purchased your WHMCS license directly from WHMCS Ltd. rather than from HostGator, you should contact WHMCS directly for all issues.
Here are some online resources you can use for WHMCS: