OP's tree is pretty small. I don't think there is any risk of over application. In fact, gypsum is recommended to mitigate salinity issues in soil. And it totally depends on circumstances. If you have calcareous soil to begin with, you likely don't need gypsum. Or, if you're on acid soil, calcium carbonate would be a better choice. The key is ensuring that your trees have enough calcium. I've read that ratio of Ca to K should be north of 10 to 1 for max brix (essentially making Ca a super macronutrient). Lack of Ca also produces softer flesh, which in carambola manifests itself as fruit that has little to no crunch.
Usually growers in my area (west broward) don't need to supply calcium. But my situation is very unique. I have a layer of black muck several inches thick, which is the result of a decade worth of ultra heavy mulching. Compost / muck has its own set of advantages and disadvantages, too much K in relation to Ca being one of the bad points. I had the curious experience of observing all the fruits in my orchard drop in sugar content gradually over a 10 year span as the mulch decomposed and feeder roots started to hang out in the richer muck layer.
You could probably also throw the Ca to K and Ca to Mg ratios out of balance by applying too much K or Mg. This can be detected as a color change in fruit flesh (darker orange usually) and softer flesh (eg, internal breakdown in mango). Unless you're in super ca rich soil, you probably don't want to apply K without Ca.
One of the disadvantages of gypsum is that it doesn't seem to stick around for very long. Har, recommended a less soluble form of Ca called soft rock phosphate, which I've been applying as well
Jeff, when you say "a few pounds a year" for a small tree, do you mean? 10, 20, 30lbs? and how often should it be spread around the tree, all at once? or a couple of pounds every two week? I just planted my first one last month so I want to make sure it gets what it needs to prosper. Thanks in advance! XE