Gas guage stays on full

The sending unit has either become jammed, its electrical terminals are corroded, or it is dead and needs replacement. It's in the tank, so you'll have to get access to it (most likely by dropping the tank). Once you have it out, you can inspect it and see whether it's dirty connectors (clean them with baking soda and a toothbrush, plus electrical contact cleaner), or damage that necessitates replacement.