You may want to do a soil test for your landscape, taken in several locations around the property before you make any additions. You can submit it to the UD Soil Testing Lab (http://extension.udel.edu/dstp/ ) for a small fee. Usually gypsum is not recommended in landscape soils unless you have high sodium (salt) soils.
Your shale based soil probably would benefit from the addition of some organic matter, in the form of mulch or peat, or spent compost soil from a mushroom or other operation. This will also help with drainage. It is difficult to increase the organic matter and drainage in clay soils, it can take several years.
Thank you for contacting Delaware Cooperative Extension,