Many modern motherboards use an additional 4-pin power plug to supply extra power for the CPU. Some specialist mobos require a main socket of 24-pins and also use a 6-pin additional power plug. It suggests to me, and as Gary's also mentioned, you need a PSU specifically for that Gateway motherboard.
How much effort do you want to put into this? Do you still have the original (blown) PSU? If "yes" and "yes" and you're prepared to open up the old unit, then you might be able to modify the connectors to get it working.
First of all, bear in mind that opening a PSU, even a "blown" open can be hazardous. High voltages can still be present, even several days after it was last plugged into the mains. Trace the wires that terminate in that 6-pin plug. With a bit of luck they might just be 3 black wires (0v) and 3 yellow wires (+12v). If they are, then cut off the old 6-pin connector with a reasonable length of cable still attached to it, obtain a "Y" type molex power splitter cable (this gives you male and female 4-pin molex plugs), and then splice the appropriate wires together. You should now have an adaptor lead which can be plugged into one of the 4-pin molex power plugs coming out of your new PSU, with the other end going down to your Gateway 6-pin motherboard connector. Additional power for the CPU without any mods to your new PSU.
Good luck!
ROGER - G0AOZ.