Previously, gateway game consoles were strictly Nintendo. As time progressed, the console that the gateway games are played on have progressed as well.
Normally, a gamer with an older sibling is introduced to their gateway game by their older siblings. In this situation, the young gamer tends to play many gateway games.
Jacob: Yo, Jackson! I just played the best game at Kyle's house! Call of Duty: Black Ops, I need to go and get a Xbox 360 so I can play that great game.
Jackson: Welcome to the world of gaming, although you could've picked a better gateway game.