yes, i personally use the pellets. seem to work well. ($5 for a 40lb bag).. others use gypsum board and break it up in smaller chunks. ive never used powder or the board but i'm sure they would both be okay.
I put down some Gypsum when I plant my seed (if planting rake and scatter method).
I do that to boost calcium levels since mine are a little lower than ideal.
I also put down gypsum on my planted beds early in spring (late Jan, early Feb) before they sprout.
That way it has some time to get washed down and in to the root area before they sprout and start growing for the year.
On my seed producing bed, I put down a healthy dose of gypsum and some bone meal + green sand, and some epsom salt + a little home made compost for a top dressing and I do that in Feb usually. In my seed producing bed I have noticed that several of my 4 prong plants have regressed to 3 prongs over the past couple of years, so I am giving them a little more help than usual to see if I can bring those 4 prong tops back.