demo (free games) can be hidden from steam account view in some cases. Install it via steam app and it should appear (maybe). not from galaxy but open just steam, install game, then in galaxy wait till it will refresh games
I installed it from the Steam app which to my knowledge, is the only way to install a Steam game.
if you restart galaxy and game does not appear then it's a bug. it should refresh all games on launch, if it's not happening even after game is installed fill bug report with steam plugin. this plugin on start should read all of your steam vdf files (manifests for installed games) and that should ensure all installed games (even demos) have to be visible. if it fails then it is definitely a bug and should be reported