G Scale (Also known as Large Scale) is the largest gauge used for Model Railways. Because of the size of this gauge, it's locomotives and rolling stock, they are more suited for outdoor areas. This is one of the gauges Bachmann uses to produce their Thomas & Friends line.
G Scale was introduced by LGB in 1968, but it never really gained popularity until 1972 when it was introduced to the American market through the Model Railroader Magazine. In 2009, Bachmann began producing a Thomas & Friends line in this Gauge which has proven to be mildly successful.