I found the solution for Windows 10 (and probably other OSes). This problem likely happens because 640x480 isn't explicitly supported by your graphics card or monitor/display. You need to set a custom 640x480 resolution, likely with your graphics card.
If you have Nvidia, do the following:
1. Right-click on the desktop
2. Click on "Nvidia control panel"
3. In the Nvidia window that pops up, add a custom resolution of 640x480, and leave the default values
The exact UI of the Nvidia control panel varies with the OS. You might need to poke around to find the right button to add a custom resolution. I recommend googling "Nvidia custom resolution" and add your OS.
I'm not sure how to set custom resolutions for other video cards.
I can't post a link yet, but you can go to the GOG forum for "Emperor of the Fading Suns" and go to topic:
Game won't start "Enumdisplaymodes failed"
I posted a few more details there. I just discovered this fix last night. Finally, after about 6 years, I'm able to play some of these games.
This fixed Final Liberation for me as well as a dozen other games.