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The term G gauge is used to describe a variety of popular large scale trains with widely differing proportions.  Technically it is incorrect.  The track, the only thing that most of these trains have in common, is identical to the old No. One gauge, having 1.77" between the running rails.  Number One scale has a proportion of 1:32 or .37" to the foot.  European train makers have used it for years.
G scale is a 1:22.6 proportion, or .63" to the foot.  On that basis, G gauge track would require 2.6" between the rails.  When Lehmann Patentwerk, a German firm, began manufacturing its LGB brand of large scale trains in the late 1960s, its engineers followed European narrow gauge prototypes.  As a result, they gave its models a whimsical look--short fat, and top heavy--that many people found appealing.  These LGB trains were scale models with excellent detail, but they looked more toylike than anything else on the market.  Of course G scale narrow gauge track was about the same as the No. One gauge track, which was already available.
Over the last 20 years, half a dozen other manufacturers have entered the large scale train field, including Bachmann and Aristo Craft, all using the same track gauge.  Scales range from 1:20 to 1:32, with steps in between at 1:22.6, 1:24, and 1:29.