Are you assuming that nobody who buys a GabeCube is going to buy a game on Steam ever again?

Is it perhaps more likely that users with a convenient box attached to their TV might want to buy more games from Steam?

Now this might be difficult to track, but stay with me. Valve makes the GabeCube. Valve owns Steam. Sales from Steam go to Valve. Users with Steam hardware play a disproportionate amount of games bought from Steam. See where this is going?

There's absolutely no difference. You can run games from other stores on a GabeCube, but most people will play Steam games. People who play more games buy more games. Just like people who mainly play Xbox buy more Xbox games.