I used to work in the Fertiliser industry some 35 years ago. Forgotten most of what i knew. However, at that time there was some additive called "Carragheen Moss Extract" used for crystal habit modification. Excuse my Gaelic spelling. I don't suppose that it is used much these days, and that now there are much more efficaceous synthetic chemicals.
My first post was to steer you towards a process technology which I know has an interest in gypsum crystal habit modification. The next step is up to you.