The mythos card ''Private security hired in Uptown' has a gate appear in the woods, but also has a headline leading to all monsters in Uptown streets or locations being returned to the cup. Does this mean that you place a monster on the gate and then remove it again? Seems a bit pointless.

Don't forget that monsters also get to move before you do the action mentioned in the card text. If a fast or flying monster with the correct movement symbol appears on the gate, it will be gone before it would be removed. Also, if there's already a gate in the woods, you'll get a monster surge that places monsters on all the gates - they won't be removed either.