You can get that exact unit on eBay (pulls, but they're tested) for around $25-$30. There are newer-style units that should work, they're available in the same price range and higher. The "P8" didn't tell me much; that designation is used by Astec for 3-,4-, and 6-pin connectors, used for sound cards, video and other stuff. With only two wires, I'd suspect that was used for the power-down to the PSU--not sure, though. Hoping the experts will jump in here...

Please post back with the Gateway model number, thanks.