Here are some links to tables posted around the Web, to describe/explain model railroad gauges and their scale equivalents:
http://www.spikesys.com/Modelrr/scales.html
http://modeltrains.about.com/od/modelrailroadtrains/tp/Scales-and-Gauges.htm
There's a lot of info in these, so I didn't cut and paste entire tables. G gauge (or more precisely, G scale) scales out to 1/20.3 to 1/24th scale (the gauge, or distance between the rails, is given as approx. 1.77 inches in some of the tables I saw).
The list at the first link shown above is more extensive than the one at the second link; it includes almost all modeling scales as an item in each scale's description.
1/32 is comparable to I scale.
Hope that helps, prost!
Brad