Man, what a heat sink that would have been! When I briefly did gutter, we just aligned the gutter sections carefully, upside down on the driveway of the job we were doing, and soldered away.
And, responding to Bill300d's post, it was traditional to solder galvanized steel gutter, too, not just copper.
It's been a while since I did gutter, but both my son and I still solder flashing when we make it up into shapes; and my son tells me he's not alone among the circle of construction workers he moves in - although I am aware that his tendency to do things like solder sheet metal, finish hammering sheathing with a hand hammer (he sets the gun so it leaves the nail sticking up, and finishes the job with a hammer, so that he knows he's hit framing), and so on is at least partly my fault. I raised him up in the same kinds of traditions I was raised up in, choosing quality techniques over "out-the-door" techniques.
Soldering remains the strongest method of fastening architectural sheet metal (flashing and the like). It's getting hard to find the solder bars; I grab them every time I see them at yard sales, where they're usually pretty cheap.
