Most people's tap water is toward the basic. Mine for instance is 7.8. The mix doesn't trend toward the basic very much over time. Gypsum doesn't lower the pH at all, it just supplies calcium. The sulfur that's included with the gypsum would lower the pH but it would take a long time. I recycle some of my mix every year and the pH of the mix is still plenty low enough. What I (and others) do to combat feeding plants 8 pH water, is to add a little vinegar to the water. If I add 1 tablespoon of white vinegar to 2 gallons of water, it lowers the pH to about 6.5 which is the approx pH of my rainwater. So IMO, don't add any gypsum or sulfer, and just add a little vinegar to your water.

Kevin, pH management of container substrates is a big subject and there are a lot of great resources out there. Relative to your water supply, the alkalinity of your water is the bigger determinant of whether you need to adjust the pH than pH alone. If edweather can adjust the pH of his water from 7.8 to 6.5 with 1 tbls of vinegar (acetic acid, which is a fairly weak acid) in 2 gallons of water, then his water is not very alkaline. I would not bother with the vinegar with what he says he observes. Here is a link by the horticulture group at North Carolina State University that does a great job of explaining what you are asking about.
http://www.ces.ncsu.edu/depts/hort/floriculture/plugs/ph.pdf

Your water provider measures the alkalinity of your water & can tell you what it is. Whether they will is another matter entirely. They usually will.

Like all soilless media, 5-1-1 has its pluses & minuses. Your success is a function of how well you manage your media.

Good luck!

Thanks ed and DWD -- I'll do a reading in a couple weeks and see what I come up with. Nice to know something like white vinegar might help.

Kevin