Sorry for the late reply, came upon this thread in my own search for a disc brake setup on my CG.
If you look carefully, the Deluxe doesn't come with stock alloys -- it has a spoked rim with a hub specifically designed to fit a disc rotor. You can install Deluxe front shocks and it's disc setup onto your GS, all you'd need are the front shock absorbers (or just the left hand side one, if you can find it) along with the caliper, rotor, etc setup. It should drop right in without any modifications, provided that the staunchion (front fork) diameter is the same on the Deluxe and the GS. I beleive the Deluxe fork diameter to be 27mm, same as the CG.
All you'd have to do is use the same yoke stem on your GS and it would fit onto your GS no problem after installation of the top and bottom triple tree clamps. As for the rim, I've seen people procure spoked disc brake rims from the kabhar (near Bilal Gunj) for as little as 2-3k, Japanese. On top of that, get your self a disc brake lever with the necessary reservoir and you're good to go.
Now, procuring said front shock absorbers is the difficult part. I've been looking for a pair myself, but to no avail. I've checked at spare parts shops for Honda, Yamaha and Ravi but so far nobody has anything of use.
I'm starting to think that the state of spare parts availability in this country is absolutely dismal. Would you beleive it, I scoured McLeod road to find these front shocks and the Ravi authorized spare parts dealer that I found, he has to buy old Ravi Storm's for 20-30k and take them apart and sell them piece by piece xD
Anyhow, if anybody had any leads or experience with this sort of thing, or even if they're selling a decent pair of front shocks with the necessary disc brake mounts, do make sure to let us know on this thread, or a PM would work as well.