Gas Guage Problem

Well, I have a problem of my own now. About a month ago, I took my car on a 30 mile trip and on the way home, the gas line started to leak. It sat around for a month with a pan collecting gas. Yesterday I went to start it to take it around the block to get it running and my gas guage started to steadily drop. It is highly unlikely that I spilled 6 gallons of gas in 3 miles. It was around 3 quarters full and when I got home it was reading between 1 quarter and half a tank. Any idea what happened? Loss of pressure screwing up the guage? Thank you in advance!

The Camry is notorious for blowing the pressure sensor in the gas line. Periodically, my gas gauge will jump from wherever it is to full, down to empty, then back to where it was. I have to monitor my fuel level based on miles traveled, knowing I can get between 410 and 420 miles per tank.