i have had 4 GG, settled them all in my coastal capital (von oranje) then with theoc, and vassalage... +12 EXP, and barracks/drydocks/airport...
At the point, i just quit the game because i wasn't having much fun.
Wanna know what is fun?
Building an island of barbarians with 20 barb GG units, and setting them up with the world-leading techs, WB them scotland yards, jails and security buildings to avoid espionage...
and the fun will really begin when their "super pirate ship destroyers" hit the board