In Organic gardening magazine in the Nov/Dec '02 Issue in an article about soil, it said that if your soil pH is above 7.0 you can add gypsum to bring it down. Can I use discarded wall gypsum board as a row cover and to change pH or would this have toxic substances in it?

No my friend that is not true. Gypsum is calcium sulfate.
Calcium carbonate raises soil pH to be more alkaline (pH > 7).

Sulfur products lower soil pH to be more acidic (pH < 7).

Gypsum has both calcium and sulfur so it does not affect soil pH. Gypsum is used mainly on alkaline soils to get calcium and sulfur in the soil.

You need to get a good soil pH test before adding any lime or sulfur to your soil.

If you want to add anything to buffer your soil pH no matter what your soil type is, just put lots of mature compost on your soil.

Aside from the pH factors, you need to stay away from adding sheetrock, wallboard, whatever you want to call it, to your garden. It has added binders and glues that hold the board together. I liken it to dolomitic limestone versus concrete. Concrete has a bunch of limestone in it, but you wouldn't want to add concrete or mortar mix to your soil just to utilize the lime in it.