This refers to an ensemble designed to resemble the character Gary, a snail, from the animated television series SpongeBob SquarePants, intended for wear during the Halloween holiday. Such a getup is typically worn by individuals, often children, participating in trick-or-treating or attending costume parties.
The popularity of dressing up as the aforementioned character stems from the widespread appeal of the SpongeBob SquarePants franchise. The accessibility and recognizable nature of the character contribute to its attractiveness as a Halloween choice. Furthermore, it allows for creative interpretation and construction, ranging from simple, homemade versions to elaborate, store-bought designs.