It takes a lot of gypsum and a long time to really have an effect on heavy clay. I know I tried and do still use gypsum as another amendment. The best remedy is to keep amending with compost and use a spading fork to help brake up the clay and mix in the compost. ( or a roto tiller if you're luck enough to own one) This works much faster, the results can usually be seen the same season in you plants and you will have better soil than with the gypsum.
We find that the rototilling causes more compaction. The clay breaks up and develops better tilth when we use gypsum with the compost. Compost alone mixed with the clay makes for some great adobe bricks. It may depend on the type of clay you are dealing with.
If you spray with Bokashi juice after you till, you have little or no compaction. That stuff is just wonderful. DH is tilling a new area, one of few places we haven't done much with. And it's good VA red clay. Between the compost, leaves and the Bokashi it's breaking down nicely. Wish we had this stuff 25 years ago. Would have made it so much simpler to create new garden beds.