If you have GG then load up global.lua and it will show you pretty much all the commands you have available.
And there's some more complex stuff in utillib.lua, physlib.lua and one other lib.lua I can't remember off the top of my head from amenmoses
Other than that you have a few commands from lua itself (mainly file handling)
But to summize, we have access to: sounds, animations, raycasting, physics detection, sprites, player object related stuff (position, angle, weapons etc), object position, scale and angles, detecting keyboard input.
Hard to list everything in brief but take a look around the forums to see what has been done should give you a better idea of what is and isn't possible... GG is not going to give you free reign over the entire game as AGK does though, it's designed to make the process easier so tries to hide a lot of the work behind the scenes (though GG is very much still WIP itself)
