Gatorboard is expensive! Very expensive! I asked about it at the local Michaels & Hobby Lobby (Ocala), but neither ever heard of it. Sounds like an online item.
I took some foamboard and sprayed that stone look paint on it. It took only a short time for the foamboard to buckle. Maybe it was just me or the paint I was using to try and get a speckled look, but be careful when painting foamboard.
I haven't tried it, but what about using the corregated plastic stuff that most of the signmaker people use? It's strong and light. I've been using it to make molds but maybe it could work for dio walls.