Windows Demo Download
This download is the free version, which gives access to multiplayer but not the singleplayer campaign. You can buy the full version of Geneshift through Steam. You can also play multiplayer for free by clicking the "Download Demo" button on the Steam page, which is better because Steam will keep it updated.
Linux Demo Download
Running ./Geneshift will open the client and running ./GeneshiftServer will autostart the dedicated server. Note that this is a demo and is mostly used for the dedicated server. You can purchase the full game on Linux through Steam. Here are some options you can set for the dedicated server:
-m maxplayers Sets the max number of players.
-p port Sets the port to host the server on.
-h hostname Set your username here to earn community credits while hosting.