Technically speaking, almost everyone makes G gauge, not true G scale, and here's why:
Gauge = the physical distance between the rails; in this case 45mm, which is consistent across all G trains
Scale = the proportion of the model to the real prototype and this is where G gets messy
Who makes what and at what scale:
| Manufacturer | Scale | Notes |
|---|
| LGB (Germany) | 1:22.5 | Created the modern G market in 1968; benchmark brand |
|---|
| Bachmann | 1:20.3 | Big Hauler series; popular narrow gauge - Bachmann Spectrum — 1:20.3 Narrow Gauge - Fn3. (see more below) |
|---|
| Aristocraft | 1:29 | Common American standard gauge |
|---|
| USA Trains | 1:29 | Common American standard gauge |
|---|
| Piko G | 1:22.5 | German manufacturer, good value |
|---|
| Accucraft | Varies | High-end live steam |
|---|
Lionel's G gauge trains are 1:24 scale - not 1:22.5 true G, not 1:29, not 1:32. They carved out their own ratio, which is very on-brand for Lionel historically.
Marklin Gauge 1 - The legendary German manufacturer has made Gauge 1 trains at 1:32 since the early 1900s, predating G scale entirely. Their Gauge 1 line is: Extremely high end; brass construction, museum quality detail. Priced accordingly - serious collector territory
MTH Electric Trains - RailKing One Gauge. The main American producer at 1:32 on 45mm G gauge track.
1:32 on 45mm track the math works out to represent standard gauge prototype (1,435mm) - which is actually the most prototypically correct of all the G gauge offerings for modeling American full size railroads.
The
Fn3 designation tells you everything - F scale, narrow gauge, modeling 3-foot prototype track. The 45mm G gauge rail represents those famous 3-foot narrow gauge lines of the American West perfectly at 1:20.3.
The official G scale standard and its origins:
1:22.5 - That is the one true defined G scale, established by LGB (Ernst Paul Lehmann Patentwerk) when they invented the format in 1968. Everything flows from them.
A few clean facts around it:
The "G" stands for Gro"ss" - German for large or big
At 1:22.5, the 45mm rail spacing accurately represents meter gauge (1,000mm) prototype track; the narrow mountain railways of Switzerland and Europe that LGB was modeling
The NMRA officially recognizes 1:22.5 as the G scale standard. That's G scale; one ratio, one origin, one inventor. Everything else - 1:29, 1:20.3, 1:24 & 1:32, are manufacturers who came along later (with the exception of Marklin), used the same convenient 45mm track, and attached themselves to the G label for marketing purposes even though their ratios don't match. Strictly speaking they are G gauge trains running at non-G scales.
The irony is that LGB itself, the creator and true keeper of G scale is modeling narrow gauge European prototypes at 1:22.5. So even the authentic standard bearer is narrow gauge! Which is part of why the whole "G scale" label became so loose and commercially convenient for everyone else to borrow.
Because manufacturers can't agree on a single ratio, the term G scale is technically a misnomer. Depending on who built it, a G model could be anywhere from 1:20.3 to 1:32 - all running on the exact same 45mm track. That's why serious hobbyists call it G gauge, not G scale.
@Iron Horseman is one of the most serious and knowledgeable hobbist you will ever meet.
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Just don't spend your milk money. 
