The plasterboard will dry but if it has swollen as you imply it will not shrink back. The only real cure is to replace the damaged section of PB. I would make sure that the tiles are actually still firmly glued to the PB and not just wedged in place by the grout. If a repair is not possible and matching the tiles not possible as long as the tiles are secure you may just have to put up with some wider than ideal grout lines.

Richard