In a showerstall, drywall might not be the best choice for the replacement. And if it is drywall, it might be well to change it just so there are no future problems. The material to use is cement board. It installs like drywall but uses slightly different materials that will stand occasional dampness. If the surround does not go to the ceiling, you can save the upper drywall by cutting it off at the top of the surround and installing the cement board below that.