korell: Just download a new SVN DOSBox to replace the DOSBox that you currently have. Well, depending on how you have your DOSBox set up that is. I personally have a DOSBox for each DOS game so that I can make them all fully portable. Others use a single DOSBox instance shared for all games.
Here is one of the SVN DOSBox sites.
http://ykhwong.x-y.net/
Magmarock: What I meant was can you just copy over the SVN files to the gog dosbox thing?
I've never tried doing that, but what I would do is delete the GOG DOSBox files (but keep the conf files somewhere as you'll want to look at them later.
Then I'd extract the SVN DOSBox to where the old GOG one was.
Next you'll want to check out the default .conf file that the SVN build generates. I can't remember if there is one in the archive, but if you run it then it will generate one (in your User\AppData folder I think). There is also a command you can enter into DOSBox to create a .conf file within the DOSBox folder but I can't remember it at the moment. DOSBox wiki should have the info. But anyway, yes, in the new SVN DOSBox .conf file it should list all the new settings such as 3DFX, VSync and others. Add those lines to GOG's .conf files for the game and then put them back where they were.
Sorry that my details are a little bit vague in places, but I don't actually have the GOG version (yet, anyway) as I've been playing the Shareware version of Descent (which I only set up yesterday), so I don't know the location of DOSBox and the .conf files for the GOG version of Descent 1+2.
I'm actually considering using DXX-Rebirth myself, for the better graphics, fixes, addons, etc. The first shareware level played fine and kept the feeling of the original.