G scale or G gauge, also called large scale, is a track gauge for model railways which is often used for outdoor garden railways because of its size and durability. G scale trains use a fixed track gauge of 45 millimetres (1.75 in) to accommodate a range of rail transport modelling scales between narrow gauge (~1:13‒1:19‒1:20), metre gauge (1:22.5), Playmobil trains (~1:24), and standard gauge (~1:29–1:32).

A garden railway or garden railroad is a model railway system set up outdoors in a garden. While G is the most popular scale for garden railroads, 16 mm scale has a dedicated and growing following especially in the UK. Model locomotives in this scale are often live steam scale models of British narrow gauge prototypes. 16 mm scale track, the same gauge as O gauge is probably now more popular in the UK than G scale.