calcium

Gypsum -- hydrated calcium sulfate -- has little effect on pH. The same goes for calcium chloride, which is often used to prevent blossom end rot on tomatoes.

The above "experts" recommend mixing the oyster shell or whatever with your compost well before application, this can vastly increase the availability as well as the permanence of this added mineral. Otherwise, lime products often need annual reapplication.

As to quantities, I remember Andersen talking about no more than 500 pounds of a lime product (somewhat equivalent to oyster shell I think) per acre at one time. I think this is very roughly a pound per 100 sq. ft., not much at all compared to normal liming rates. This shouldn't really upset your pH, Arden mentioned that calcium doesn't do that much to pH compared to magnesium, potassium and sodium. He's actually seen pH drop after application of calcium, probably thru replacement of more alkaline elements.
Egg shells provide calcium to your garden while coffee grounds provide a high content of nitrogen to your garden. Calcium and nitrogen supplements (egg shells and coffee grounds) will help keep your garden soil and plants healthy.

The gypsum question might revolve around the sulfate content, which will always have some depressing effect on many microbes (sulfur is a natural fungicide). Also some gypsum is artificially produced, tho most available up here is naturally mined and therefore acceptable for certified organic input. I would only use a bit of gypsum along with other calcium sources...but I am always tempted to include some because sulfur can easily become deficient in my high rainfall climate. The ideal time for application is before planting and it should be tilled or disked in. This application rate was "moderate".