This link may also be of some interest, Sarah, as it contains more info on the Ernst Paul Lehmann company and Wolfgang and Eberhard Richter prior to the launch of LGB in 1968:
http://www.tcawestern.org/lgb.htm
When the Richter Brothers decided to build a model (toy?) train system that would be big, chunky and work outdoors as well as in, they chose the 45mm track gauge that was (as Nick says above) already existing for Gauge 1 models - but they decided they wanted their trains to be larger than the 1/32 scale Gauge 1 models, so they chose to build them to a (nominal) scale of 1/22.5, which made the 45mm gauge track scale out as representing the one-metre-gauge lines used in a number of European Narrow Gauge railways. Since their launch in 1968 with a version of the very Stainz loco that you have (though the first ones had a different and rather noisy gearbox mechanism and are commonly called "growlers" here, or "Heuler" (howler) in Germany), LGB - Lehmann Gross Bahn - is still the largest part of what we now know as "G Scale" trains, at least in Europe.
From the start, LGB have had what they called the "Radius 1 Rule" - which means that EVERYTHING they ever produced MUST be capable of running round their minimum-radius (R1) curves, which are approx. 2 ft radius or a 4ft circle - of course some very large locos and rolling stock may look a bit silly doing so, with massive overhangs front and rear, but the important thing is that they are technically capable of it.
There are regular discussions here and practically everywhere else that model railways are discussed about what "G scale" actually MEANS - and the answer is not a simple one - it's not really a "scale" at all, but the one common factor is the 45mm gauge track - beyond that, the actual scale of the models can be pretty much anything from 1/29 (used for some big American stock) to about 1/18 - and it's even arguable that models made in 7/8ths scale (about 1/13) could be considered as part of the G scale family too.
Jon.
EDIT: Martino posted while I was in the middle of typing this, so apologies for any duplicated info and opinions but we're all pretty much saying the same thing!
