I keep launching Gabriel Knight 3 and it keeps trying to launch and then it will say trying to Launch in 640-480 mode and then it will blink and say Game requires 16 bit support. On my previous computer I went into the Graphics card setting and then just adjusted it to display at the required Display settings. This computer has an Nvidia Graphics card and I can not figure out what to do. I figure someone on this forum has this game that knows what to do for this problem. Please Help. I know there is a thread somewhere created like 5 years ago, but that thread will not help me.

I just had the same issue - found the solution:

https://www.gog.com/forum/gabriel_knight_series/gk3_wont_run_requires_16bit_color_mode

Now to get the window to go full screen...

Which I found and posted -
https://www.gog.com/forum/gabriel_knight_series/resizing_gabriel_knight_3_resolution_solution