https://www.g1mra.com/

The Gauge One Model Railway Association

Quote from their website:-

Gauge One requires a track gauge of 1¾ inches (44.45mm) which is generally referred to as 45mm. In practice this means Gauge One models are from one to two feet in length (300mm to 600mm); 50% longer than O gauge models and 50% shorter than “3 ½ inch”. This in-between position means the hobby is influenced by both model railways and model engineering; at its best it takes the concern for prototypical system accuracy of the former and the authenticity of the latter..