I do sometimes wonder at the fascination of guitar builders with Tru-Oil and then trying to make it do something it is less useful for like making a gloss finish.
I think big part of the attraction is that it is a very forgiving product that can be applied by hand. Not every DIY guitar builder has the luxury of spray equipment or even experience with spraying, so Tru Oil has become known as a reliable, almost fool-proof finish option.

FWIW, Tru Oil on its own is glossy. One of the components is polyurethane and, based on my own experience, would say it is not a high solid form of poly that is used (like in a satin poly). I don't think it's a case of a product that's being used in a way that it isn't intended, but one that just works for lots of people who happen to make guitars as a hobby.

You're right that a shiny gunstock is not a desirable thing for hunting. If nothing else, you don't want sunlight reflecting off of it alerting deer or whatever to your presence. But when Tru Oil is applied to gunstocks it is typically done in a minimal number of coats, not with 30-50 coats like on guitars. When applied in many many coats, it naturally has a gloss to it which can be greatly enhanced with the wet sanding and polishing that we read about.