You might have gypsum board. It's an exterior type of 'drywall' that's normally meant to be covered up with some sort of siding. It's still used today but not that much. The downside to its use is it doesn't reduce 'swaying' in relatively high winds. That's why OSB and plywood-type sheathing is used on homes, especially in hurricane, tornado, and high wind areas. With the swaying, you get nail pops and such in drywall if screws aren't used. You'll likely get cracks in the drywall too if a stronger sheathing isn't used.
The homeowner probably had extra gypsum board and decided to use it in the shed. If it's regular drywall (which it can be), I'm surprised it lasted as long as it did in the weather in the shed. Post up a new Post when you find out what it is. (This post is getting buried!) Highlight your new post with info from your original post (because our memories are short), and tell us what you found. Then, if you have any further questions, you can ask. Or, just tell us what you found.