My fuel gauge does not read correctly. Is it the sender unit or the gauge?

Faulty sender units are common to some GM vehicles, but generally speaking will start malfunction at the same fuel level every time due to wear at the sender grid.
Also, you may get a fault trouble code which is usually an indication of a faulty sender or circuit.
A malfunctioning gauge esp. on some GM vehicles will be stuck and not move at all or be off the scale altogether.