The motherboard has been designed to be reverse mounted, and the internals of that case really don't support more than a single-slot video card:
http://support.gateway.com/s/PC/DX200/1008689/1008689iv.shtml
So, you really can't get a video card with good gaming power into it. You can get a single slot card, but it won't give you substantial gaming performance. A Geforce 210 is a decent low-power single slot card. A Radeon 5450 is too.
Other than that, you'll need to upgrade (or rather standardize) your motherboard, case, and power supply in order to get any real gaming performance.