IBM's (now Lenova) website lists 20 'types' of 300GL, each with a 4-digit number. Over those 20 types, there seems to be 9 different video setups (depending on type).
Do you see a number anywhere? Look for a 4-digit number... something like 6263, 6338, 6591. I know there are several types of 300GL. Look around the computer, front, back, sides, information on the screen when you start up. Surely they've hidden this number somewhere, we've just gotta find it.
If you can't find the number, the only other choice would be to download and install each of the 9 different video drivers until we find one that works. They'll either give you an error while installing, or they'll do what the driver you've installed now is doing. It's a sloppy way to do it, but if we can't find that 'type' number, it's about the only thing we can do.
Here are the 9 drivers for Windows 98.
Oh, and most of us here won't give up that easily