"粉饰用石膏粉" can be translated as "plaster powder for decorative finishing" or more simply as "decorative plaster powder".
Here, "plaster powder" refers to the powdered form of plaster, which is commonly used in construction and decoration. "For decorative finishing" clarifies its purpose, indicating that it is used for aesthetic purposes, such as smoothing surfaces or creating decorative textures. If the context clearly implies its decorative use, the simpler "decorative plaster powder" would suffice.