Both come from gypsum. Plaster of Paris sets in a few minutes and is used for thin finishing and mouldings. Gypsum plaster has a controlled setting time and is made for a full plaster coat on a wall.
Gypsum plaster for walls must meet IS 2547 (Part 1), which asks for a setting time of 10 to 30 minutes. That time lets a crew mix, apply and level a coat of 6 to 20 mm.