Gateway MB? You're hosed. Those are unsupported by anyone including gateway. They are custom built for gateway by random, unheard of chinese assemblers and you normally can't find a replacement 12 months after they come out, even from the gateway parts store.
You really need to get the vid card into another system to make sure it's ok. If it is, it's MB time.
In the mean time there is one other option since that PC was shipped around and you don't know what it's original home was like.
-Fully disassemble it,
-clean out everything with canned air,
-then reassmble making sure the MB holes are lined up correctly so they don't have shorts to any circuit traces.
If the MB slips a little it can produe intermittent shorts ot the chassis.
And don't rule out the power supply. Gateway isn't known for using quality supplies. Blow the dust out of it also with the canned air. Stick in another PSU if you have one.