Remember that this test should be performed with ALL of the spark plugs removed from the heads and the ignition coil primary harness disconnected (I usually stick a small light bulb into the socket to act as a load for the ignition module). It may also be advisable to pull the fuel pump fuse so that your cylinders aren't getting drenched in gasoline.
Also, remember that after performing this test "dry," you might want to add two tablespoons of motor oil to each cylinder, right before taking another compression reading. If the pressure raises dramatically, it means that your rings are worn.
I second Gearhead's notion that the
balance between the cylinders is what is important. Typically more than 10% variance, and it's time for a rebuild. Perhaps you should purchase a service manual for your vehicle, as most of this information is contained therein.
FWIW, 120psi seems awfully damn low. Are you sure your gasket on the gauge's hose is sealing properly, and that you're opening the throttle fully?