I bought the game on release here and I like it very much. With the demo they also published a new update for the game improvement. If anybody cares, Kronos does obviously not update through GOG, but uses its own game launcher to check for updates and install them. Dunno if one can backup a fully updated version of the game or if there will be an updated installer available at some point on GOG.
As for the demo, I checked their homepage and for English the demo seems really only available through Steam at the moment. For German the demo is available without Steam here: http://battle-worlds.de/demo
or as a direct download here: http://www.crimsoncow.de/bwk_demo_setup.exe
I have not tried the demo, maybe it is multi-language. Anybody interested should try! At least I suppose the English demo might come without Steam on their homepage as well soon.