Gypsum primarily works for certain kinds of clay (sodic) which are not all that common. I think they are more so in Philip's area but we don't know where valthm441 lives and whether the same might apply there. Adding organic matter is the general recommendation for improving clay soils. Sand is not recommended because to change the soil texture enough you would have to add an impractically large amount to see any improvement. If you are in the United States your local Extension office would know if gypsum might be worthwhile in your area.
