Someone please tell me does the Greenside of the greenboard gets put on the outside???? I just read where the greenboard facing goes towards the wall. Messed up by family relatives. Doing job or redoing job Thursday? Thank You TG

Hi, I'm replacing an "in the wall" medicine cabinet in my bathroom. It is the normal style that has a mirror that opens on a hinge. My problem is, the original medicine cabinet is smaller than the standard sizes carried at Home Depot. I would modify the wall, but there is framing in the wall that exactly fits the original size. Are there vendors that sell non-standard sized medicine cabinets? Could I cut through the framing without causing structural instability?....then put in a standard size? Any help would be appreciated. Thanks Kurt P.S. I threw the original out when doing demolition so I can restore it. :(