Hi,
There is no available training on the GSS as far as I'm aware - even the course content for the ACE module doesn't cover it.
The only thing I've found apart from the official product documentation is a single chapter in the Cisco Press book "Designing Content Switching Solutions" by Zeeshan Naseh and Haroon Khan.
However, the key to really understanding how the GSS works is understanding how DNS works - if you have a thorough knowledge of DNS then picking up how the GSS distributes DNS requests becomes straightforward. So, in the absence of GSS training, DNS training would be an excellent substitute.
HTH - plz rate if useful
Andrew.