Desperados 3 and System Shock remake got a demo. https://www.rockpapershotgun.com/2020/05/27/play-demos-for-desperados-3-and-system-shock-right-now-via-gog/
I can't imagine they are exclusive to the platform, but they are featured on the front page of GOG current sale. Perhaps timed exclusive? It is still 3 weeks until Desperados release (guess who is counting!) so plenty of time to bring it to steam as well.
EDIT: Yup, Desperados is pretty much a higher budget Shadow Tactics, and that's all I wanted. The pausing Showdown Mode does make things a bit easier, but that might mean I might try to fight a way out instead of immediately reloading. The Demo consist of tutorial, first mission (half tutorial half genuine mission) and a taste of mission no. 3. Runs great, looks great plays great. I love the end-mission detailed replay, and customizable difficulty settings. Can't wait for the release. Mocap improves presentation a lot (as well as more detail and some nice little visual touches), though it is somewhat at odds with lack of facial detail, and blocky hands.