I think the GG is one of the better GPs...no surprise - I like playing Imperialistic...in my current game Im just about enter the modern age and I have accumulated a total of 8 GG (a big thx to the Great Wall)
The first 2 GG were settled in my most developed military production city, where I added the Heroic Epic
The 2xGG makes the equivalent of an early West Point (plus a few beakers and culture with Rep & Sistine)
The next 2 GG became Warlords 1 medic & 1 elite cavalry with Leadership to give even more GGP when they kill
The fifth GG also became a Warlord assigned to the my first Caravel
Nav I & II + Morale
much easier to prove that the world is round
this result in quick privateers that can outrun annoying frigates and course more trouble

, plus every XP point the privateers earn is 2 GGP (perhaps 4 GPP with GW)(smashing Galleys and Caravels becomes much more fun)
GG nr. 6 and 7 were saved for Military Academies in 2 other production-minded cities, and one of these 2 cities got West Point
so I have a HE/WP city + a ½HE/WP city
you could make a 2xWP/HE city, but I like to have more well-experienced units than few(er) insane-experienced units
Im not sure what to do with GG nr. 8 (most of the GGP to him was gained with Privateers)
probably another Military Academy
Im lining up for a Domination or Conquest victory, so the production boost would be nice
When the carnage begins more GGs will turn up, and they will become Warlords to help the onslaught on track