The smell of wet gypsum

Question

Not strictly chemistry, but where does the distinctive smell of wet gypsum come from?

It is not like wet earth from geosmin.

Answer 1

It's from weathering (chemical, microbial, other) of gypsum ($\ce{CaSO4\cdot 2H2O}$) and mostly arises from the evolution of $\ce{H2S}$.

That said, there are numerous other sulfur-containing compounds involved in gypsum decomposition which can/will contribute to the aroma.

See Gypsum Drywall Odor Landfills Jan 2002 Update from the EPA for more details.

Geosmin is a terpene and doesn't contain sulfur so while subjective descriptions of "earthiness" and the like may be warranted, they do not arise from similar chemical processes involving sulfur-containing compounds.