I would be interested in buying Gabriel Knight and certain other games if I could download the original DOS versions (with bugfix patches applied, of course).

The reason for this is that it's easier to emulate DOS than Windows on machines that aren't running Windows natively. A Wine (Windows emulator) instance doing just about anything is fairly resource intensive, especially compared to DOSbox or something similar, and it's an unnecessary step to emulate Windows just to run a repackaged DOS application.

It would be fairly easy to do this, I think, since these are the files GOG is brewing its Windows version from. If it were an option to simply download those instead of the Windows installer, surely at least a few people other than me would appreciate it.

Ehm what GOG is distributing for GK1 is the original DOS version, bundled with DOSBox into their own installer.
If it's the GOG installer that bothers you (the new one does not always run smoothly in wine) you could extract the files you need with innoextract.

I suggested extracting because he seems to want to play it on Linux, and I've had some problems with running the v2 installer under wine (kind of an Windows emulator) in the past.

Yes, thank you. I've figured it out. I guess it would be nice for GOG to have a description of what their installer does — just something as simple as "This runs through DOSbox" would suffice.

I'm surprised they don't package the game for Mac and Linux versions of DOSbox as well. Maybe they only have the license to sell on Windows? Or maybe they just haven't got around to doing it yet.

Unfortunately GOG does not support Linux at the moment, they decided to support Mac and Win 8 last year instead (*yuck*).
I don't know why they don't package it for Mac, perhaps they don't want to do that as there was a native Mac port back in they day. Maybe they are trying to get the rights/data for that.
It is often a bit frustrating for Linux gamers that GOG does not make the game files available without them packaged with their installer. But I don't if demand for that is all that high.
Oh and there is a GOGMix with a list of all games that use DOSBox and a category for it on the GOGWiki, so it is not that hard to find out if a game uses DOSBox. And there are lists for ScummVM too.

This game is slightly harder to get going on it's own, but not that hard.

It basically mounts an ISO image as a CD on D drive (just look for the largest file). And then it has a few files on C drive.

Like all Sierra Games, editing RESOURCE.CFG is key to making sure all the Sound Options and paths are correct. This way you can put all the files into a single directory and the game will work just fine :)