What else is in the GH booster you have? Almost always there is magnesium, and often potassium, sometimes other minerals, too.
If adding all of these are OK, and you can add enough GH booster to get the Ca where you want without over dosing the other materials, then go for it.
If you know that all you need is calcium, you might look for calcium chloride.
I may be talking nonsense here, or this may be of no use to you at all. I used to "farm" common pond snails to feed to my green spotted puffers. the snails needed large amounts of calcium to build healthy shells, so I needed to add calcium to the water. I found that the easiest and cheapest way to do this was to drop in holiday feeding blocks. You know, the ones you defiantly would NOT use to feed your fish when away, but are never the less sold for that very purpose buy practically every retailer.