a large frame Vespa will handle a gravel road about as well as most street bikes tho with 12" wheels it's not its strong point. They will also do freeway duty as well tho at under 300cc's (depending on the model) there is not a lot of juice to spare above 70. What they do exceptionally well is to dart in and out of city traffic in the 30-60 range carrying you, a passenger, and a fair amount of groceries in style and comfort. The bikes are very well built and other than some teething problems when the EFI's first came out quite reliable as well. All that said, if I lived in Idaho I would not sell a GS to get a GTS. I'd buy the GTS and then pick up a 3-5 yer old KLR to scratch the gravel itch. Unlike the GS it will be happy to live under a tarp in the back yard and if you drop it you'll only tick it off. One bike is a nice Idea but there really is no one bike that does everything well. If the separate heated garage and Leno-like sable is out of scope remember, Craig's list is your friend.
“many a trip continues long after movement in time and space have ceased”- Steinbeck