The Windows ME disc won't have a suitable driver on it to get more than 16 colours. It only has a standard Microsoft driver (16 colours only) so you can at least get Windows up and running with any video card until you can install the proper manufacturer's video driver.
It has to be an NVidia driver, nothing else will work. You haven't given us the information we need, which is the actual model number of your Gateway PC. We can then use that model number to find out which NVidia driver you need, because there's no such thing as "one driver fits all". It would be so much easier for everyone if there was!
We also need to know which part of the world you live in (it's not in your profile) because NVidia, like most manufacturer's, use different model numbers in different countries and often fit different components in different parts of the world. They cater for this by having regional support websites. We need to know which region is the right one for you.
If you feel competent to navigate around the Gateway website yourself, go here:
Click on the relevant country or region, then when that new page has loaded, mouse-over "Support" then click "Drivers & Downloads".
Now use the 3 drop-down boxes to select your type of PC, the series, and it's model number.