My 1970's vintage brick ranch house was sheathed with a gypsum product. Due to a leaking tiled shower the bottom 18 inches of the backside of the shealthing had some mold. I cleaned the mold. Now the paper backing has peeled from the gypsum.
I realized replacing the sheathing would be the ideal. I am trying to avoid that at all costs.
If the gypsum is basically intact, but has only lost the paper facer, I would seal it with Kilz. You should really test it for moisture content first, to make sure it has completely dried. If you can beg or borrow a moisture meter, test it and compare your reading to a known dry sample of the same material close by.