So if you ground the wire that hooks up to the gauge to a known good ground back at the tank and it pegs the gauge full then i would suspect the gauge and wire going to gauge is good...you can go one step farther by getting a 180ohm resistor and placing it between the ground and the sending unit wire going to the gauge and it should read empty......so if that works i would start focusing on the tank ground or the sending unit itself.
Next i would probably pull the sending unit out and get a small jumper wire hooked to a good ground and attached to the body of the sending unit where the screws go through and manually move the float around to see if the gauge sweeps from F to E.....it may take a few seconds for it to react so don't expect it to move real time...it depends on the resistance in the sending unit sometimes but it should move....if not then something wrong with sending unit.....if it does work properly then i would start looking at how the sending units is getting its ground signal...maybe you have corrosion around the screw heads insulating it from the sending units body and keeping it from making good contact?

interesting...maybe ive been running into some off the wall stuff :)