Gas Tank Question ?

I have a 2003 Pontiac Grand AM 6Cyl...I filled my gas tank up last week ..All the way full...Now my gas guage is staying on full all the time,,,The gas guage is working because when i start my car the gas guage does move but returns to the full side...Does the gas a tank have a float in it.....Where could i take my car to have this problem checked out.....i called the dealer and they what to take my gas take off and inspect it and charge me 350.00 dollars...I posted this Question last week to this group and i did receive alot of answers to my question but nothing worked...I sure do appreciate all of your help....Gary