Old Gas in Tank
How old is it?
Raise the back of the car up high.....remove the supply hose to the fuel pump and it should start to siphon out.....if not use a vacuum pump to start the siphon.
If the car has been sitting for a long time, there is likely a coating of varnish like film inside the tank. The good news is that is easily removed. The bad news is that the ethanol in the fuel will remove it and send it down the fuel line and clog your fuel filter on a weekly basis.
If the car has been sitting for many years, you will be better off removing and thoroughly cleaning or replacing the tank.