Your fuel sending unit is probably malfunctioning. The gas gage is nothing but an ohmmeter and the sending unit is a floating unit on a rod that changes the resistance and the level reported by the gauge itself. The gauge itself could also be malfunctioning but this is less common.
The fuel sending unit is part of the fuel pump assembly. While it is possible to change the fuel sending unit without the fuel pump, it is recommended that you purchase the entire assembly as access to it on this vehicle requires that the fuel tank be removed from the vehicle first. The labor to do this is somewhat expensive so you wouldn't want to have to have the same procedure done again if the fuel pump later needs replacement.