Gremlin Guardian is the final unique collectible Pin in Epic Mickey. It can only be obtained when talking to Gremlin Gus while Inside the Blot.
Gus tells Mickey that he earned the pin for freeing every captured Gremlin in Wasteland. Because there is no quest telling the player to do this, and it is easy to accidentally miss gremlins, it is very rare to see casual players obtain this pin without a guide. In addition, the exact requirements for Gus to give you the pin are yet unknown. There is evidence of players missing a gremlin and still getting the pin, and some players claim to have found every gremlin and somehow missed the pin.