2 gauge (also called II gauge) is a model railway gauge originally 64 mm (2.52 in), but standardised in 1909 at 50.8 mm/2 in but since fallen into disuse. The gauge was introduced by Märklin at the Leipzig toy fair in 1891. 2 gauge was equivalent to 1:22,5 scale. 63 mm (2.48 in)/64 mm (2.52 in) gauge was standardised in 1909 as gauge 3. Gauge 2 was standardised at 2 in (50.8 mm).
The European G gauge trains are built to the same scale, but with a rail gauge of 45 mm (1.772 in) (the same as 1 gauge), which is the equivalent of 1,000 mm (3 ft 3 3⁄8 in) gauge in modern railroads. As a result, the scale-gauge combination is sometimes called IIm in European literature.
In the UK, No 2 Gauge was 2 in (50.8 mm) while No 3 gauge was 2 1⁄2 in (63.5 mm). From this, it follows that G gauge is sometimes, albeit rarely, referred to as 3m. a 2 in (50.8 mm) gauge of standard gauge locos gives a scale of 1:28.25, not so far off the 1/29 used by some manufactures with 45 mm (1.772 in) gauge track. In the grand tradition of model rail gauges often being narrower than the full scale equivalent (00 scale being the classic example) this is deemed perfectly acceptable.
Reference
Model Railways and Locomotives Magazine Vol. 1 No 8 August 1909