I've used NutraSoft OP Gypsum,
"I've used NutraSoft OP Gypsum, before but at a lower application rate and saw improvement to my heavy-clay soil. This time, I applied at a higher rate and after lawn aeration/overseed. I do not expect to see immediate results, know I'll see soil improvement next Spring when I detatch. The product application is super easy. I highly recommend wearing eye and respiratory (N95 mask) PPE and in low wind conditions, because it's dusty (I also wear long sleeves & pants to keep it off my skin)."