Have a careful inspection of the board itself for silk screen (printed on the PCB) markings which may be the Board model number/series. It's shame that you can't even get it to post because then the BIOS/Post information will probably tell you more about it. I'm not familiar with Gateway computers but *if* it's anything like older Dell ones (I really don't know), be very careful about using a standard connection PSU. Although some old Dell PSUs came with standard looking connectors, they were actually wired in a proprietary way - this meant plugging in a standard PSU could fry the board. Perhaps you have access to the PSU that was in the Gateway PC as well?
If you have the full Gateway model details, their website will probably have manuals covering the motherboard and PC as well (along with downloads for the OS that was installed on it in those days).
TYVM for your time and advice. Unfortunately, some motherboards are not clearly marked for identification. I was able to resolve the situation, however, by using your advice.
Glad all is now well I agree with your sentiments on the motherboard marking situation and I have seen components where OEM part number stickers have been placed over mnaufacturers identification numbers (not on a Gateway, I've no experience with them).