Hmm, railroad gauge is measured from inside-to-inside of the rails. In the US it's a nifty logical number like 4'-7 1/2" (0.o). The UK is similar, but, some of the other European countries are not. IIRC the Soviets, in particular, had a dissimilar gauge than the Germans, and this was for strategic reasons, so that rail cars from one side would have to stop at the border and have the contents trans-shipped.
G scale is not uniquely American, IIRC, as it's "Garden" scale, approximately 1:28 on a narrow gauge (either 1 meter or 3 foot gauge) track.
I gauge (either "I" as in Inca, or I as in Roman numeral 1) is described as being from 1/36 to 1/32 scale
Which is important a rail wheels are around 24-30" (60-75cm) in diameter. So the 1:28 G gauge wheels would not be in scale for 1/35. The rail profiles are going to be not quite right, too.
This is why some On3 (O gauge using HO gauge track as 3' gauge) modelers will hand lay O gauge track at HO width so as to not have problems with the wheel rims being deeper than the track height.
The rail guys think these things out, as having your rolling stock derail in a curve and wipe out the scenery makes for a bad day.