Originally Posted by
xenologer
Sounds like the ratcheting pawl is not grabbing. Could be old dried up grease keeping it from moving...............
..............could also replace the whole shifter pod with a cheap friction lever, those pretty much are bulletproof.
Shimano lubed those shifters with white grease which turns to gum fairly quickly. Pretty straightforward to pull apart and clean. I have picked up old mountain bikes cheap that didn't shift. 1/2 with cleaning solvent and some black moly-d grease solves 95% of the issues.
Your GT is a good bike. It'll last for many years to come. It's definitely worth keeping in good shape.