Actually clients NEVER reports to GUP. They are always connected to the SEPM & SEPM will re-direct them to their local GUP.
You can get the GUP list through the SEPM console, this article can help you:
How to search for the clients that act as Group Update Providers ?
http://www.symantec.com/docs/TECH96094
In case you want to check defintions distribution there Conent Distribution Monitor tool might help.
The SEP Content Distribution Monitor tool helps monitor GUP health and status as well as general content deployment. This tool is a lightweight, stand-alone tool designed to be run directly on the Symantec Endpoint Protection Manager (SEPM) server, and should return a graphical display of the content distribution status
http://www.symantec.com/docs/TECH156558
To verify if clients are receiving defintions from GUP can run the Sylink monitor tool.
How to confirm if SEP Clients are receiving LiveUpdate content from Group Update Providers (GUPs)
http://www.symantec.com/docs/TECH97190