Gas pipes aren't cast iron, they're black iron or galvanized.
I'd stick something (pole, string with weight) down there to see how deep it goes. Then fill it with water, you should be able to estimate how much it should take, once you know how deep it is. If it doesn't fill up then it must be connected to a drain... probably your sewer line. Have somebody flush the toilet while you listen to the hole. If it's your sewer line you should be able to hear it and may even feel a rush of displaced air.
Plan B - replace the blue plastic cup and fugetaboutit.
