OK - lets stop beating up the OP about tank size at the moment. A rock that is stuck in the fish's throat is a bit of an emergency. You have a few days to work at it or you won't have a fish left.
If you can see the rock lodged in the throat, and the fish cannot eat, you need to manually remove it. That means anesthetizing the fish so it will allow you to work.
Ideally, you want as real anesthetic (like FinQuil), but clove oil is prob what you can get your hands on in a hurry. This is the instruction:
Sedate with Finquel or Clove Oil
Once the fish is anesthetized, you need to open its mouth & grab the rock with long thin tweezers & remove it.
If the fish can still eat, occasionally, it can work the rock out by itself. So you can wait a few days. If it can swallow the rock, then it will likely pass out the other end. But a rock that is totally stuck will need your help (or a vet) to remove it.
Once you get rid of the rock, you should remove your aquarium gravel. As a rule, goldfish should have gravel that is either small enough to pass through the system (1/8") or less, or too big to be swallowed (say 1/2 to 3/4").
And yes, a common needs to be in a pond eventually ... they get to be 12-18" long ..... <A 20 is only big enough for a fancy ... not a common or a comet.>