At the very end of Galaxy Configuration of Darklands, the game will not finish installing. Config error 26563720. What is this and how do I resolve it?

I don't know what the definition of a "dosbox game" is. However, I have (and play) Dragon Age Origins, Divine Divinity, Temple of Elemental Evil, Baldur's Gate and Neverwinter Nights.

Neither of those games run on DOS like Darklands. By dosbox games I meant to say "games which require to emulate MS DOS with dosbox" usually you don't need to worry about that because GOG gives you the game imbibed on DOSBOX.
Can you try to download the game without the gog galaxy client (click on your game in your library and look for "Download offline backup game installers") and see if it still doesn't work?