Bought the game for a dollar. Still have my original CD and a copy of the game via Steam (which haven't worked on my computer ever since Windows 10, worked just fine on Windows 8) and I am doing a replay of the entire series and would like to play Invisible War again along the way as well. However, after downloading from GOG, I am greeted with the error "Failed to start DX2main.exe!". What gives?

Just bought the game and am having the same issue. Yours is the only post on this particular error I could find. Did you ever fix it?

Same here, and then GOG Galaxy crashes.

You have to install the unofficial patch : https://www.ttlg.com/forums/showthread.php?t=147423