I know this post is old, but I found another option. I have a 1440p screen myself and Geneforge 5 was way too small on native resolution. Steps to fix it were:
Enable integer scaling in the GPU driver settings (tested on AMD GPU)
Calculate half of your native resolution width and height
Set the game to skip the settings screen on start so it doesn't mess with the settings file you are about to modify
Open the "Geneforge5SettingsAM.dat" file in a Hex Editor and find the position where the resolution width and height is stored. I used HxD and it has a number search function to easily find it.
Change both values to your calculated ones and save.
Now you have a sharp 2x upscaling of the game 🙂