I wish I could answer your questions abelthorne, but even though I have a GOG version of Deus Ex and downloaded the GOG version of Revision, then installed it over the GOG version of Deus Ex, the mod still tried to open Steam. :o
I honestly don't know what going on here. Maybe because I am using the classic (non GOG Galaxy) version of Deus Ex but I think this mod should work in a fully DRM free manner; that means it should be classic install friendly. GOG please fix it or get the mod's devs to fix it please!

This seems specific to your installation, I can assure you that I can play DX + Revision from GOG without Steam or Galaxy.

Now that is strange indeed. Perhaps I need to look at my startup files and see if Steam is running something in the background because as soon as Revision starts, it immediately tries to open Steam and download Deus Ex.
EDIT: There is a Steam Client Service listed in Services tab. I will uncheck it and see if that makes a difference.
EDIT Mk II: Yes! That fixed it!
Now to your questions abelthorne, I uncovered a help file. Sorry I can't recall where I got it, Mod DB or Steam forums maybe? Anyways I will post it's advice here:

"You can play with the submods easily!

Either change the revisiondefault.ini in your system folder to revision.ini or copy the revisiondefault.ini and paste it again and rename it revision.ini (if you don't have that file). Now instead of replacing default files with New Vision or Shifter, make folders for them inside Deus Ex instead.

Some people experiencing crashing, you need to have hdtp and New Vision in its own folder in Deus Ex (Just like Shifter), not inside Revision. When you have those ticked in Revision it will look for it in its own folder and we don't want that.

Here is the fix for that open the revision.ini go look for the lines

Paths=..\Revision\NewVision\Textures\*.utx
Paths=..\Revision\HDTP\Textures\*.utx
Paths=..\Revision\HDTP\System\*.u

and replace them with:

Paths=..\Revision\System\*.u
Paths=..\Revision\Maps\*.dx
Paths=..\Revision\Textures\*.utx
Paths=..\Revision\Sounds\*.uax
Paths=..\HDTP\Maps\*.dx
Paths=..\HDTP\Textures\*.utx
Paths=..\Shifter\System\*.u
Paths=..\Shifter\Textures\*.utx
Paths=..\Gore Mod v2.0\System\*.u
Paths=..\HDTP\System\*.u
Paths=..\NewVision\Textures\*.utx
Paths=..\System\*.u
Paths=..\Maps\*.dx
Paths=..\Textures\*.utx
Paths=..\Sounds\*.uax
Paths=..\Music\*.umx

Do that for DeusEx.ini as well. So it should look like this:

[Core.System]
PurgeCacheDays=30
SavePath=..\Save
CachePath=..\Cache
CacheExt=.uxx
Paths=..\Revision\System\*.u
Paths=..\Revision\Maps\*.dx
Paths=..\Revision\Textures\*.utx
Paths=..\Revision\Sounds\*.uax
Paths=..\HDTP\Maps\*.dx
Paths=..\HDTP\Textures\*.utx
Paths=..\Shifter\System\*.u
Paths=..\Shifter\Textures\*.utx
Paths=..\Gore Mod v2.0\System\*.u
Paths=..\HDTP\System\*.u
Paths=..\NewVision\Textures\*.utx
Paths=..\System\*.u
Paths=..\Maps\*.dx
Paths=..\Textures\*.utx
Paths=..\Sounds\*.uax
Paths=..\Music\*.umx

Or replace Shifter with Biomod or GDMX or whatever.

There may be an error that appears with one file in "HDTP\Textures" called "HDTPanim.utx"

and one file "HDTPItems.u" from beta 2 in HDTP\System.

I also get a similar error when I install beta 3.

Copying the files into both the Deus Ex and HDTP texture and system subdirectories fixes that for each of them, "DeusEx\System\" and "DeusEx\HDTP\System\" ; "DeusEx\Textures" and "DeusEx\HDTP\Textures\". Now, I have the "in-game" menu and the new characters textures."
Hope that helps you abelthorne.

Well, I haven't tried yet since I posted here but basically, I'd like to avoid tweaking the setup like with the original DX. I seem to remember (from the "cracked" Steam version) that Revision includes its own launcher similar to Kentie's but I suspect that it'd need a bit of tweaking anyway (not sure about the OpenGL renderer version included, not clear if it uses HDTP or not...).

If it needs various editing in config files, it's a bit annoying (I don't use Shifter or similar balance patches).

You could try asking on Revision's Mod DB page. I have seen it's devs (and other people very knowledgeable about changing settings in Deus Ex) answer all sorts of questions.

Well, even if I'm not going to play it these days, I've just installed DX + Revision to check by myself. So: Just needed to install the game + Revision. Revision indeed has its own launcher that manages the same kind of options as Kentie's, no need to install anything else or tweak config files. Revision includes HDTP (though the characters models are disabled by default) and New Vision.

As for the OpenGL renderer, it's a bit more complicated. According to a post on the Steam forums, it supports OpenGL but the renderer isn't installed by defaut. I've tried copying the up-to-date DLL in Revision's System folder: can't select it. I've tried to add the .ini + .int files of the original rendered in the folder and then I can select it but it appears twice in the list. Selected one of them and it seems to work. Not sure why it appears twice. I've downloaded the soundtrack installer on Revision's website, installed it (not sure it's complete, it installs only music for the intro and Duclare's castle) but when I launch Revision, the intro music doesn't sound different. Not sure what I did wrong. Might be related to the way I launch Revision, or might need to adjust a config file (I haven't found an option in game to toggle original / new music).
EDIT: there's an option in the game sound menu to use either the original Deus Ex or Revision soundtrack but it's already set to Revision. i'm not sure it includes a complete soundtrack and the music pack is unreleated to this option or if it's buggy.
EDIT: turns out I was confused by what is in the music pack: I thought it had an alternative version of the Deus Ex theme that is played on the main menu. In fact, it replaces the music of the cinematic introduction. It works well, I just didn't understand what it included.