chalk and water
Gypsum mortar is obtained by mixing equal amounts of water and plaster. This type of dough allows to make the so-called "shaving" or "smoothing", ie a very fine plaster for interiors.
For this type of plaster it is not recommended in damp environments such as kitchens and bathrooms because the plaster is strongly hygroscopic and would tend to swell.
This type of mortar is very useful in the case of plasterboard because it is used to fill the panels.
Sometimes, to reduce shrinkage and avoid cracking after drying, small amounts of very fine sand or Hydrated lime are added.