Enclosing masonry fireplace, required clearance

I've read the code regarding masonry fireplaces and the minimum clearance from combustibles, which is 2" on the front and sides, and 4" from the rear face. My question is, is type x drywall considered combustible? I'm trying to keep the wall as close to the fireplace as possible. I'd like to use metal studs laid flat against the brick, then type x over. I found one reference to gypsum being considered "limited combustible", but no answer as to whether the 4" rule applies. Is this sort of thing left open for interpretation by the inspector? If we're being extremely literal about it, if gypsum is limited combustible, then it isn't combustible.

Just got off the phone with the county and they said type x gypsum is not considered combustible. So I can glue it directly to the brick if I wanted. I'll be going the metal stud route though. Also, it sounds like 2" clearance from the chimney portion is required from framing members, but the sheathing can but up against it.