Mugita Sokio

I think what gives GUN the feeling is that it takes place in the wild west (which is dying), and that the mechanics regarding guns, characters, and encounters are something to behold once you get into them. It’s a non-D&D system that primarily uses d10’s if I’m not mistaken (I hadn’t played it, but only seen gameplay of it).

I know of one, that being GUN by Abbadon, which might be a good idea.

I know that Ironsworn is decent for this sort of thing, as that’s singleplayer and single GM capable. I’m not sure if it’s high fantasy, though. I think you could also do something with any old D&D game up to 3.5e if you change up the rules a bit to ensure it becomes single player and single DM capable, but I’d have to take a look at how people could do this.