I don't know what this GOG Launcher bullcrap is, but I want to be able to play the games offline. The game description specifically promises I can do this, but practice shows otherwise. The game doesn't start when I choose to remain offline. What's going on?

The offline version of the game is under the name "ddtrilogy" in the directory that you installed the game under.
Just go to the game's folder and you'll see it there.