"I've heard that molding made of gypsum"

You probably mean plaster, nothing like gypsum drywall.

Before drywall saw wide use as finished walls, plaster walls had moldings 'run in place' on the wall.

You can purchase plaster molding sections to attach to the wall now instead of running them in place.

A search on 'plaster trim' will show sources.

Lightweight plastic is available that works very well for crown molding. It can be found with a rougher molded surface to mimic painted wood. And there are varieties with paper coverings to look like wood.

MDF is also made, but requires two or three people to install since it breaks very easily.

Pine is, IMHO, a terrible wood for trim. Poplar is much better(for painting)and maple is as good, just usually more expensive. Maple has the advantage of being stainable(or dyed) and used with just a varnish/etc. finish.