I would download a linux live cd, and burn it, and boot from it, that will eliminate if it is a software issue or not. If you still have the dark spot it will be hardware, and really there are several possibilities as to the issue. As it is an all in one, it will use the ram as video memory, and the cpu actually has the video chip integrated into it. It could be the lcd inverter card, or the lcd itself. or the power supply is weak, and not supplying enough power to run everything. So to help eliminate them try this if it occurs with the live cd, it is hardware. Try running a monitor or tv from the video out, does it have the same issue? if so you can rule out LCd, and Inverter card. If not, then look to those, with the obvious choice being the Inverter card, first as it runs about 25-50 dollars, compared to a LCD with a bulb or led set going bad. AIO computers generally use a desktop cpu, but can have a mobile chip, This has a desktop I3-2120 But it has laptop sodimms for memory, it could be either at this point, look at youtube video on how to get back cover off, your unit has 6gb ram, so it has 1 4gb and 1 2gb dimm, remove one of them and boot it up to see if issue still there, if it is, swap it with the other dimm, is it still there? if it is, at this point it is either the power supply or the the cpu/gpu, and at a guess I would lean to power supply as cpu's rarely fail without something like a heat issue or a power spike. If you get lucky and the live disk doesn't show the issue, look at updating the video drivers, get those from Intel and not from gateway, because gateway drivers will be down level, they always are.
