Soils typically don't lack CalMag, in my experience CalMag related issues are related to watering, or excess fertilization. People tend to overwater, locking out the CalMag, or they over-fertilize and lock it out. In some rare cases, your source of water itself can cause CalMag lockouts. Point is, CalMag issues are a bit of a "meme" with growing plants in general. It is very rare someone actually has Calcium or Magnesium related issues, the causes are often poor watering practices or excess fertilizing.
Your top dress plan sounds great, I love Dr. Earth products and use them when available. Good shit to use, if you following instructions on the bag you'll be good more often than not.
As long as it wasn't too much gypsum and/or epsom salts, you should be fine adding them. I preferred foliar feeding with epsom salts when I used them, Epsom Salts are great but only as a foliar feed, they don't do all too much in the soil itself. I wouldn't look to add gypsum/Calcium until I knew for sure I had an issue.
Calcium, Magnesium, and Potassium are all part of a "trinity" so to speak. When one is in excess, the others are locked out. Conversely, when one is deficient, it effects the ability for the others to be absorbed.
Try to think of growing plants like making a pasta sauce, or some other cooking recipe. You can always add salt to your sauce, but you can NEVER take it out. It is always preferable to err on the side of caution, and not add too much, as opposed to adding too much and having to start all over.
Regards.
