Ya, there is usually an "x" when they specifically WANT you to play the sound of a muted string. You'll usually see that with full dead strings though. In this case, your technically not "supposed" to play the dead sound because the G chord doesn't use it. But in this case for strumming, muting the note is the easiest way.

Technically, you can do it by just fingering each string. You do this a lot more in classical or any time you aren't using a pick. You can hit the Low E with your thumb, then at the same time pluck the bottom 3 strings with your index, middle, and ring fingers. That way they are all played simultaneously without a dead/muted string thrown in there.