Several different uses have been proposed for recycled gypsum drywall.  The most common ones are use in the manufacture on new drywall, an ingredient in Portland cement manufacture, a soil amendment, and in compost.  See the links below for more detailed descriptions.

New Drywall --  Many drywall manufacturing facilities use post-manufacturer and/or post-consumer scrap drywall to produce new drywall. Portland Cement -- Gypsum is an ingredient in portland cement manufacture.  It is added to the cement clinker before the ball mill. Land Application -- Gypsum provides a source of sulfur and calcium to crops.  Gypsum can also improve the drainage and texture of clayey soils. Compost -- Scrap gypsum drywall, or the paper separated from drywall, is often added as an amendment to composting systems. Other uses that have been proposed include animal bedding, flea powder and for construction products.

Scrap drywall in a roll-off box at a construction site