iOS is the gatekeeper. AirPlay is how iOS plays media wirelessly. iMessage isn't a gatekeeper because it isn't popular enough in Europe. It comes with iOS, but

iOS is the gatekeeper. AirPlay is how iOS plays media wirelessly. iMessage isn't a gatekeeper because it isn't popular enough in Europe. It comes with iOS, but most European iOS users don't use it for enough of their messaging for the regulators to give it that classification.