To test the guage and cable, you can clamp the cable in a drill and spin it while someone checks to see if the guage is reading. If nothing, problem is in the cable or guage. If the guage reads something, start by pulling out the plastic gear where the cable enters the tranny as this is a fast and easy check.
that plastic gear is driven by another one. As you are pulling the one check to see if you find any plastic of another color as unfortunately it is often the inner one that fails.
Let us know what happens please.
Good luck.