My Hinterberg frame has a long table running from end-to-end on the backside. This is where I can lay a pantograph pattern, which are sewn from the back of the machine. (My machine has an extra set of handles on the back for pantograph work.) I think quite a few machine frames are set up this way, especially if the longarm setup does not come with computerized quilting. Computerized quilting options include pantographs which can be digitally loaded -- no paper pattern -- so you can work from the front of the machine, using the pattern displayed on the computer monitor as your guide. So, I expect the Gammill comes with this back platform for laying down paper pantograph patterns.
One thing I will say is that a cutting mat laid on my Hinty's platform would be too low for me to be comfortable cutting.