Gypsum for Gardening v. Food-Grade Gypsum

This has been asked time and time again, but I wonder if Gypsum from the garden section of your hardware store is really all that different. The guaranteed analysis of Gypsum on the 30# bag says 92% gypsum, 8% Calcium, Sodium, and inert materials. To a novice, this seems pretty... gypsummy. Does food-grade Gypsum found in Asian markets promise an even better guaranteed analysis, or should hop heads everywhere head to their favorite tool shop and nab a 30# sack of lawn gypsum? Is local homebrew store gypsum the biggest conspiracy to brewing since Belgians saying light candi syrup is really all that different than table sugar? Is putting stuff bound for the lawn in my beer going to kill me?!

Thoughts? Because 20$ for a lifetime supply of Gypsum seems like quite a bargain.

Have your LHBS get it in for you by the pound or buy a sack of food grade terra alba with a bunch of friends or approach a local brewery/brew pub and ask them if they will sell you a couple of pounds. Why deal with the uncertainties of 'inert materials'?

Sulfur-dioxide scrubbers in coal-fired power plants produce very large amounts of gypsum (http://www.fgdproducts.org/FGDGypsumIntro.htm). I'd bet there's a pretty good chance that's what's going into those bags of lawn gypsum and you'd be taking a pretty big gamble about what else could be in there.