You can run any scale outdoors although I believe that G scale is really designed to this type of operation. I had a friend who had an outdoor S gauge layout that ran fine. He had to be careful that nothing he would leave outside would rust from snow or rain but aside from that, he had no problems.

Smaller scales (like HO or N gauge) might be problematic since a blade of grass could derail the whole thing but I suppose if you mow your lawn regularly, you’ll be okay. ;o)

Geof