On a ringed engine that size, 2 or 3 gallons would be barely enough to run it in let alone wear out the ring. Engines don't wear out from just sitting so I'd go along with the stuck ring. If you can't free the ring by soaking with fuel and flicking it over for a while then the next thing I'd do is to take out the piston and gently push on the ring with a finger nail to get it to move in the groove. Sometimes they can get gummed behind the ring and this doesn't let the ring expand outwards all around. Usually they'll expand at the ring gap a little so put fuel in the gap and push the ring inwards at the gap. This will slowly allow fuel to get behind the ring and dissolve any gum.