Hi,
Software vendors like IBM normally have a 'certified' list of supported data sources. This means at the time of release of the software they tested and documented certain versions and combinations of the softwares. Time goes by and all software evolve and sometimes the certified lists are not updated. This my be the case for your combination of Cognos and GP.
This does not mean that the two pieces of software are not compatible or will not work for you. As long as you use a standard GP odbc or jdbc driver to GP 6.x you should be good to go. There might be some niggles between the information schema between GP 6 and GP 5 which Cognos BI 10.2.2 has not adjusted to.
I guess the only way you will know is if you setup and connect Cognos BI 10.2.2 with GP6.x!
Test it, try to break it and let us know.
Regards,
F