Rocks? Gypsum is fairly soluble. But I agree that it takes time to fully dissolve. I note that even when you add gypsum to cold beer to check if it benefits from additional sulfate, it takes time. I add my gypsum to my brewing water in the HLT and that too "takes time".
The 'Rocks' thing is quite odd. My gypsum is a fine powder and it never agglomerates into little rocks. Is this a brewing product or something else?