Save Gus' House is a quest given by Gremlin Gus when Mickey enters the World of Gremlins.
After painting in the pipes and shutting down the Thinner, Mickey can go up to where the Thinner was flowing and paint Gus' house back in. Happy to have his house back, Gus will fix the Village's Windmill and will note of the good deed. Mickey can then enter the house to get the Bronze Pin in the Treasure Chest.
The Gremlin Village Windmill that is fixed after saving Gus' House
This quest is optional and is not required to progress the game. However, if the quest is not completed, Mickey will not be able to access the Projector Screen shortcut to the European Boat Ride, as Gus will not fix the windmill, making it impossible to reach and rescue Gremlin Ditto.