I don't have a G400, I have a Banshee but I did experience the same problem with Unreal Tournament (it played in a window not full screen). I found out it was becuase the hardware acceleration (in my case Glide in your case D3d) was not running. The window game you get is software mode. You need to enable the OpenGL or D3d for your card (I haven't found a D3d driver for my card yet, hopefully you can find one for yours).