Top and bottom? Sounds like the game is running at 1920x1080 and the monitor isn't changing resolution. And how do you display a 1920x1080 image on a 1920x1200 screen without scaling or anything? Gonna be some kinda of border.

If you were able to run the game at 1920x1200 that would solve the issue. But support for that resolution might be hit or miss in some games. 16:10 aspect ratio monitors fell out of common usage closer to 15 years ago-ish.

You might want to post on the sonic g forum, or do some research to see how well the game supports 16:10. And whatever your GPU is setup to do when it's not able to run a game in native resolution or a non-native resolution is in use. As the monitor switching to a non-native resolution may not yield the desired results either.

thank you for your help!yeah I'm running in 1920x1200 setting btw.
I think the game just doesn't support 16:10