I emailed the wall surface question to the Gypsum Association a few years ago, after discussing (ok, arguing) the point with architects; this was GA's reply "As far as I know, gypsum board is not considered to be nonabsorbent. This code requirement is generally referring to the wall surfacing materials such as tile or plastic panels. This is a typical application where water-resistant gypsum backing board is used as a substrate for ceramic tile."
Notice water-resistant gyp not moisture resistant (green board). Of course, what determines "hard" - hammer blow or finger push? Your interpretation here.
While on the subject, do your require the floor to extend upwards as required by 1210.1 or do you accept a base material?
IBC 2006 1210.1 Floors. In other than dwelling units, toilet and bathing room floors shall have a smooth, hard, nonabsorbent surface that extends upward onto the walls at least 6 inches (or 4 inches in 2009 IBC).