Is it worth using Gypsum?

Gday again.
Once I've finished my retainting wall I'll be putting some turf down. I've had the backyard excavated and in most areas I'm down to shale or half way thru it. I'm wondering if its worth putting gypsum down. I used it in the front yard to breakdown they clay, but I'm thinking theres no way in hell thats its gunna break down the shale.
I will be running a coule of ag drains under the turf and putting a 100mm layer of turf underlay and then SirWalter on top.
Hopefully that'll do the trick

Cheers