Corrosive primers will leave salt crystals in the barrel and gas cylinder after shooting. Humidity will cause moisture to collect with the salt, which can cause pitting. But this all takes time, so if you see rust after shooting, it probably isn't the ammo.
Cleaning with water is a good first step, followed by normal cleaning, as HSMITH said. Just like the blackpowder guns. Be sure to take the gas system apart, too, including the valve that switches from semi-auto to single-shot (grenade launcher) mode.
I'd advise shooting up all that old corrosive stuff ASAP and buying new non-corrosive ammo. Corrosive is a PITA in gas-operated guns, so much to clean. I shoot it in bolt guns, and carry a little bottle of windshield washer fluid for initial cleaning at the range. That stuff won't freeze.
Regards.