Could be a bad gauge, or the wire between the gauge and sender unit could either be shorted to ground or "open", depending upon what type of gauge system it is.
Try this, at sending unit, disconnect wire that comes from the gauge, and note position of the gauge needle. Jumper the removed wire to a good ground, and again note the position of the gauge needle. (Obviously, look with the key switch ON both times.)
Should read "full" one way and "MT" the other. (Some gauge systems are designed to read "full" with the wire disconnected, and some are set up the opposite (more common) way, and the gauge reads "MT" with the wire disconnected, NOT sure which system you tractor has.