Yes; and winter too. Gold fish can tolerate cold temps as long as there is plenty of depth below the winter ice. They can eat algae and whatever else. I have a pond with a 24/7/365 running stream and another pond with no pump except when using it for irrigation. It has tons of underwater plants but it was originally almost free of plants when I initially added three comets that turned into 30 or so. I did feed the original three sometimes but it is out of site and often out of mind and three years later I almost never feed them.

I have had a comet and two other goldfish in a stock tank with no filter for about a year, and they're doing fine. I do feed them, though.

I kept goldfish outside spring fall and summer in large utility containers from hardware store (two per container) for years before I built them a pond. Each container held maybe forty gallons. I brooght them in in winter. However they have only started breeding since they got a pond never did befoe so I guess they are happier. (possibly I did not have them boy girl boy girl but in pairs of same sex was impossibly to tell!)