There are a couple of posts on here that seem to start out asking what is considered to be "in play," but they quickly change to answers about what's considered to be "in Arkham." There are Mythos cards which direct you to remove certain monsters that are "in play." Are monsters in the Outskirts considered to be "in play?" To me, "in play" means "on the board, in front of players, etc." Each post that I can find in my search for this answer always turns out debating what is in Arkham and what is not. I think that "in play" is different than "in Arkham." Is there a definitive answer to this? (Outskirts = "in play")
As the rules indicate that when closing a gate, monsters matching the gate's symbol in all areas including the outskirts are returned to the monster cup, I would say that this is returning all the monsters "in play" to the cup. That is different that "in Arkham" which is defined as being in a stable, unstable, or street location in Arkham, Dunwich, Innsmouth, or Kingsport, or in the sky.
The outskirts are not "in Arkham" but they are definitely "in play" as they have an effect on the game.