HITMAN WOA is one of my favorite games ever made. I have played about 600 hours across the three entries that comprise it and that number is only growing.
The game's biggest problem, always, has been its DRM and release method, both of which GOG could address. I don't expect it to happen soon given IOI are still very actively supporting the game, but one day we need the GOG final edition, one button, no online requirements, buy it all edition of this astounding game. Fan projects have already successfuly adressed the server issues, it can be done.