Drywall – How far out off the wall framing do gypsum panels for the ceiling need to be

Why?

You are going to have to seal and finish the edges where the new wall meets the existing walls and ceiling. To do that, you are going to compromise the area of the existing drywall. Why not trim out a channel in the existing drywall the width of your new studs so that you can get good, tight firm attachment points.

The problem with butting framing up against drywall is that drywall is not very solid. Flexing of the framing can tend to crush and crumble the drywall between the new framing and the old. While this is not a load bearing wall, you do want it solid and stable, especially if you are going to hang any shelving or other moderate loads. Also, if you remove a strip of the old drywall, you can make sure you are attaching the new framing solidly into the old and not driving to close to the edge of a stud.

While you might get away with it, there is little reason to do it. If you insist on going that way, I suggest you use construction grade screws rather than nails. If you use nails, the very process of nailing may increase the likelihood of crushing the drywall underneath and creating a looser joint.

Do you need the fire resistance and sound-deadening of drywall, or just want the look?

You might consider something else that would look quite similar but not have the weight.

I put textured paintable wallpaper over wood paneling, like this textured wallpaper. Once painted, it blended right in with the drywalled wall next to it. Nobody knows it isn't drywall, even standing right next to it.