Gateway 507gr power issues...

Unplug the four-pin 12V ATX cable from the motherboard, and see if that makes it power up any better - it probably still won't work entirely, but see if it gets your cooling fans spinning. If so, you probably have a bad motherboard, with something shorting out the 12V power.

Is the 12V processor input plugged into the power supply¿ I will assume that you only removed the PS and it's connections and reinstalled it without removing any other connections. There may be more connections than I may have anticipated.

I assume you are using a new power supply.
Now unfasten everything from the motherboard (except video) and try to power it up. That is, disconnect the hard drive, CD, and any add-in cards. If it still doesn't power up, I would try to swap out memory. If you still have no luck, you will probably need to replace the motherboard.

Just a post to verify the popular claims that bad capacitors in the power supply cause it to fail and also kill the motherboard. I found "NEW" motherboard and power supply for $30 each; and what else is there to say. Listen up: Intel augsburg motherboard, and the Athena 400watt power supply, both for 70 bucks. Amen... PV.