THE SCALES AND GAUGES
There is often confusion between the 16mm and G scales due to their similarity and also with respect to the track gauge (measured between the rails).
Perhaps looking at the evolution of the scales will clarify the present situation. 16mm to 1 foot scale was originally devised for modelling 2 foot gauge prototypes e.g. Festiniog or Lynton & Barnstaple Railways, the models using 32mm i.e. Gauge 0 track. However, 45mm i.e. Gauge 1 track can also be used for a wider gauge - this also permits some of the G scale items to be run. 16mm scale is a ratio of 19.1 : 1.
G scale however, originated on continental practise, generally metre gauge railways, at a scale ratio of 22.5 : 1, running on 45mm (Gauge 1) trackwork. There are slight variations to scale in some of the larger G scale proprietary items, but more importantly, on narrow gauge railways there is a terrific variation in the physical size of the real thing. Therefore with a little care, much of the 16mm scale equipment can be used as G scale and vice versa.
Gauge 3 is a standard gauge i.e. modelling 4'-8 1/2" gauge railways and is based on 2 1/2" gauge track. Working back from this the scale ratio is 22.6 : 1 i.e. virtually identical to G scale and in practical terms well suited to mix with 16mm scale.