What is G scale?

G scale was introduced by Ernst Paul Lehmann Patentwerk, when they introduced their innovative
indoor/outdoor "LGB" brand trains in 1968.

A typical G-scale model train is 22.5 times smaller than the real train it represents. By comparison, an
HO-scale model train is about 87 times smaller than the real train it represents, and a Z-scale train is
about 220 times smaller.

Although 22.5 sounds like an odd figure, it's really quite sensible.

That proportion will to make attractive and realistic models of real European and American narrow gauge
trains (1000 mm) that would run on traditional 45 mm gauge model track. ("Gauge" is the distance
between the track rails.)

The math is simple:
Distance between the rails of meter gauge track - 1,000 mm
Divided by the distance between the model rails - 45 mm
G-scale proportion - 22.5