Hello,
I bought some game off GOG in the past year and recently acquired a quite great MS-DOS 6.22/Win3.1 machine. I would simply like to know if lets say GOG's Fallout would still work under MS-DOS?

Unfortunately no. GOG sells the Windows version, it is not a DOSBox wrapper.

and none of the MODS for fallout 1 work in DOS anway AFAIK.

i tried running the installer in win98 and even that wouldn't work.

I may be wrong, but in theory, one you've installed a GOG game - a game using Dosbox at least - on a recent OS, you could be able to pick the folder content and use it on an old OS, except if the .exe launcher has been modified.
For games using ScummVM, well, as the original launcher is not required by ScummVM, it seems that GOG don't include it in their releases.

The original version of Fallout requires a P90 and 32MB (for DOS).
So I don't know if a rig that came with Win 3.1 would even be able to run it anyway.

Thanks a lot for all your answers!
@Navagon: I meet the requirement, Pentium MMX 200 with 96mb of RAM

Have you got it running under DOS? Because I also assumed that they packed the Windows installer only.
Or is GOG's Fallout 2 a DOSBox title?
I still use my original big box copies to play F1 and F2.

GOG sells the Windows version, not the DOS one, of Fallout 1 and 2. They may be playable under Windows 95 if one first installed in on an XP box then copied everything to the 95 machine, DOS or Win3 is a no-go though.
Some of the DOSbox games may work in a similar way unless DOSbox is doing something funky like accessing a CD image and the game believes it's reading data from a CD drive.