I used to have a thinkpad with ATI display technology*.  I figured out this solution for doing VJ sets.  In your control panel / display / setting / advanced you need to switch the svideo out monitor to the primary screen and make your other monitor be the secondary monitor.  Then adjust the sizes (i.e. make the svideo monitor as small as possible), I suggest matching  the desktop background color with your Gem background colour, and hide the task bar.  I use Gem + CVS and it is a bit buggy when it comes to giving too many commands to the Gem window I suggest give it

[offset x y]
  [dimen x y]

then create your window, make sure it is where you want it.  Then distroy your window.  put in your

[border 0]

then create your window again and start the rendering.

* I recently aquired a nvidea based pci video card and have no problem at all with gem on the second screen, I think its an ati issue.