Shacknews outlines a workaround that can get Supreme Commander 2 to run under Steam for those still receiving a "game is unavailable" error when trying to launch the RTS sequel for Gas Powered Games, saying that verifying your game cache should clear this up. This can be accomplished by right clicking "Supreme Commander 2" in your Game Library then clicking Properties > Local files > Verify Integrity of Game Cache. After the verification is finished you should be able to launch the game.