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.
They are usually good or there is NO question they need replacing oke: They can be picked up over the counter at any tranny shop. They are designated by color. There is a good chance yours will be a light green but there are several colors. This is why I said look for another color plastic when pulling it out which could indicate a failure of the inner plastic gear.
Of course in all of this, look for the REASON for the failure, not the SYMPTOM. If the gears are stripped try turning your cable by hand. If it is too tight or kinked or such it could make it drag too much and thus would have caused the weak plastic gear to strip.