The GC must be in space. It's the best explanation - we know he has advanced technology. We know he's confident no one can strike him back, and space is pretty hard to hit. We know he can see everything going on, so he probably has telescopes/satellites. If you look closely at the scenery during his cutscenes, you see that his window is full of stars and black areas - which is consistent with space. His ability to control the meteor probably has to do with being in space. The reason why he's so rich and can hire everyone is probably because he can create gold with his advanced technology.
We also know we'll be going back to the modern science institute in LF3, and I bet you anything they will tell you they saw some weird new planet or star and it's really the GCs base and they will send you on a shuttle to go up there.
Well either that or he's underseas and we will send a 2 man strike team of marylis and petula to save us from the GC.