Boyz,
What scale is a Gn15 mine critter?
In other words, do you use a 1:20.3 figure (Engineer) on the loco?
Anywhere from 1:24 to 1:20.3 is typical.
You do know of this outfit, yes?
http://forum.gn15.info/index.php
The GnATTERbox
Gn15.info’s discussion board
Depends on what scale you call “G”, and which one you choose to model. 
As I understand it, Gn15 is 1:22.5 scale running on H0 track.
I think 1:24 scale on HO works out well too. Plus you have the added benefit of many vehicles available in this scale and building in 1/24 is a very simple conversion on a common ruler- 1 inch = 2 feet.
Definitely check out the gn15 forums listed above,
Dave
It should just be a math problem (15"/.64959 HO Gage in inches = 23) but the answer is none of the above so 1:24 or 1:22.5 are the closest.