My advice would be to solve the gnat problem. Then you can bring the bins indoors and they'll do great inside. We often keep bins in our laundry room.
The solution to gnats is simple: just make sure that no food is exposed to the air. For me, that means lots and lots of cardboard. When you add food (coffee grinds, banana peels, apple cores, etc) first dig a nice little hole in the cardboard bedding. Drop the goodies down in there and make sure to thoroughly cover it all up. When you look in the bin, you should only see cardboard on the surface.
Also, consider adding a lid if you haven't done so. Just poke little holes in the lid so air can breathe through.
If you went and just added a few inches of very wet cardboard pieces to your bin, right on the top, I bet your gnat problem will go away pretty soon.
