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

You want this patch for widescreen support as well.

Try the unofficial patch already mentioned from here:

https://www.moddb.com/mods/deus-ex-2-visible-upgrade/downloads

install the full version then the beta.

Also try DXVK:

https://github.com/doitsujin/dxvk/releases/tag/v2.5.3

Download the archive dxvk-2.5.3.tar.gz

Open archive with something like 7Zip or winRAR and copy d3d9.dll to the Deus EX - Invisible War\System folder

For me the Unofficial patch got the game started but everything was over to the right of screen. Using the patch and DXVK fixed that.