This is your bible:
http://www.mechbox.com/
The G&G out of the box is actually not that bad. You don't need to do anything to it until end of the season where I'd take it apart and shim the gears properly, regrease the gears, do the Angle of Engagement mod, and maybe toss a new spring in there depending on it you want a bit more "oomph", say an M110 or M120 depending on your compression (NB: Not necessary however you might as well while you're in there since it's a ~$15 upgrade, also the field limits are limits, not goals. I'm happy with anywhere between 380-399 FPS on .20's for an outdoor gun and I bring it down even further with .28's). Also while you're in there and have the hop up and barrel out might as well clean the barrel properly with silicone oil on a wipe and change out the hop up bucking to something like a Guarder Clear, or Prommy Purple or whatever the cool kids are using these days.