I had a similar problem with an El Camino I had. The rubber gasket had come off the gas cap and was stuck in like a backwash in the filler tube. The harder I pumped the worse it would throw the gas back out at me. I took a garden hose (flexible) and beat the gasket down into the tank and filled up.

My reward for my effort was now that when I drove, the gasket would suck over into the gas pick up and starve me into a stall. Ultimitely I had to drop the tank and go in to retrieve the gasket.