what is the cause of gasoline in the oil?

Its probably running rich and loading up with gas. Or it might be getting to many short runs. The engine runs richer when cold. If the oil never gets up to operating temp it can't burn off the excessive gas or release the moisture in the oil. A good way to check for blown head/gasket is to run engine and watch the coolant overflow for bubbles. Also look for oily residues in coolant. If it is bad enough you will notice white smoke from exhaust. Any one or all of these are symptoms.

Gas smell in the oil on a old car = bad rings. It probably smokes especially when accelerating. Factor in a rebuild or engine replacement if you negotiate a price.

Agreed with above^. Bad rings.

That is one possibility and real easy to check for. Do a compression test wet and dry and you can confirm or deni that one.

This is a common problem. I have a freshly rebuilt '87 that had the same problem, the O2 sensor harness wire breaks and grounds out on the outer shielding wire and shorts to ground causing a super rich condition. Also the fuel pressure regulator diaphragms are notorious for leaking and oil gets sucked into the intake from the FPR. Bad rings generally don't cause a fuel smell from the gasoline from my experience.