I would pull the motherboard battery for one minute with the power supply unplugged. Replace and try powering up. If you get a post screen, you'll be asked to enter the bios by pressing a key. You can save and exit, or set the boot order first. Try a different power supply first. Oem ps are cheap low end units; some are only 250 watts. If the new power supply doesn't work, leave only the ram stick nearest the cpu installed (uninstall the other sticks) and unplug the hardrive. If the board still won't post, you'll need a new gateway board to use your original windows operating system or you may decide to part it out and get a new system. Here's a couple of dells that were on sale recently:
http://configure.us.dell.com/dellstore/config.aspx?oc=sdcm101&cs=04&dgvcode=ss&c=US&l=EN&dgc=BF&cid=7420&lid=197374&acd=10549103-4328635-
http://configure.us.dell.com/dellstore/config.aspx?oc=SDCM102&cs=04&dgvcode=ss&c=US&l=EN&dgc=BF&cid=7420&lid=197374&acd=10549103-4328635-&URL=http%3A%2F%2Fconfigure.us.dell.com%2Fdellstore%2Fconfig.aspx%3Foc%3DSDCM102%26cs%3D04%26dgvcode%3Dss%26c%3DUS%26l%3DEN%26dgc%3DBF%26cid%3D7420%26lid%3D197374%26acd%3D10549103-4328635-&ACD=10549103-361116-FWagjgnj.
Take your time troubleshooting your gateway. I'm not suggesting a new system yet, but it doesn't look good.