i just bought a 1990 ford probe and the gas gauge does not work. Was wondering what could possibly be wrong. Please help.

 

I know you said the gas gauge doesn't work, but what is it doing , stuck in one place , stay on empty/full ? Each one of those symptoms could have a different cause.
Could you be a little more specific?

 

The gas gauge is stuck on empty.

 

These are some of the causes for the gauge to stay on empty.
The wire from the fuel tank sending unit to the gauge is separated / open some where.
The float has come off the float arm on the sending unit in the tank.
Gauge faulty (not very likely).
There are usually two wires going to the sending unit in the tank, ground, signal return to the gauge.

If you ground the wire at the sending unit that goes to the gauge the gauge should read full. That will prove out the gauge and the wiring. If that is OK the sending unit will need to be removed to confirm the float is still attached , If so, sending unit is defective.
If the wire to the gauge when grounded causes no reaction , you may need to get into dash to ground the wire at the gauge to confirm gauge is good or not.