How do I find where the oil is gushing from?

"Sounds like a main oil seal. If you have the engine steam cleaned and then inspect for the leak afterwards you should be able to see the exact spot that the oil is ""gushing"" from. You have ruled out the two obvious places that oil can ""gush"" from ( drain plug and filter) and when gaskets like what you find around the oil pan and rocker covers (or valve covers) leak they seldom ""gush"" and tend to ""weep"" oil. That leads me to believe that either the front or rear oil seal has failed which would definitely cause a ""gush"".

Although an oil seal is not an expensive part, the repair is quite involved and labor cost would be high."

Go to a oil change place and have it put up on a rack. This is not something you should procrastinate about.