I guess I started this in more than one way so better offer a bit more info.
The Gn15 name was thought up to describe the Sidelines range of kits, which Steve Warrington and myself were developing back in early 2000. They were originally conceived as being 1:22.5 scale, using 16.5mm gauge track to represent 15 inch gauge, so the Gn15 term was a reasonable description. It just so happens that there were other things happening at the same time that have a relevance to this topic.
The most influential, was Carl Arendt's early experiments with minimum gauge on 16.5mm gauge track. At this time we were both members of several Yahoo groups and through discussions on the groups and directly by email, things gathered momentum. Carl's "Square Foot Estate Railway" was developing at the same time as the first Sidelines kits were being designed and both influenced each other. When his layout was complete, Carl also used the Gn15 label to describe it, quite correctly as both the rolling stock and figures were to 1:22.5 scale.
It was from these humble beginnings that things developed. The Gn15 label stuck and as more people got involved, many decided to follow a similar path, but using other scales as well. With the ease of use of 1:24 scale for those not comfortable with metric measurements, this became the more popular scale, but the Gn15 label has continued to be used. It may not be the most appropriate term to use, but it is now well recognised and does serve to bind like minded people together.
One final note. While I don't really see any problem with the Gn15 term being used to tie a community of like minded modellers together, there are times when I don't feel it is appropriate.
The first is with models being produced commercially, which deviate from the original concept of G scale (1:22.5) to represent 15" gauge equipment. This can lead to confusion for the purchaser and he may not end up with a product which is suitable for his needs. If the model is built for instance to the proportions of a 24" gauge prototype, this should be made clear and labelling it as Gn15 would be misleading. I am looking at the way I label my kits, though they still fit the original idea of Gn15, perhaps they need more of a description.
The second is when writing articles for publication. The readers of the articles will not always be familiar with the differences as we on this forum are. So describing a model or a line drawing as Gn15, can again lead to confusion and something which describes things more accurately is required. Don't forget, once things are in print, it is often taken as gospel.
Within the realms of this forum, we can be relaxed, but once outside, I feel we have an obligation not to mislead those not in the know.