GK is used in the modelling of the figures, collectibles, models, status, etc. In most cases, they are used for Anime, Superheroes, Characters, or any other artworks.
GK is an abbreviation for Garage Kits. In some cases, Garage Kits are also known as Resin Kits. They are usually referring as assembly of scale model kit using Polyurethane Resin. Polyurethane is commonly used to produce rigid, durable, and high-resilience products.