How do I release gremlins in friends town??

If the town you are in has a gremlin picture in the top right hand corner of your screen with a number of gremlins that can be dropped, then you can leave a gremlin. Just tap on this button and a gremlin will drop into the town and your number will go down by one.

You are only able to drop a gremlin in a Springfield that has completed the first quest which is the building of the Gypsy Fortune Teller Shop. Once that is completed then that Springfield will be able to accept and give gremlins and ghosts can be smashed and the buildings then can be haunted.

If you don't see the gremlin in the top right hand corner when you visit a town then they are not done building or buying the Gypsy fortune teller yet and you can't do anything but help them out with their buildings.

Also, a town can only hold 10 gremlins at once. The good news is that the devs had the foresight to add a counter so no more guessing like during whacking day.