Transmission Not Go Into Gear problems of the 2001 GMC C2500

Transmission does not perform correctly. Transmission does not engage correctly. Most recent occurrence transmission shifted to neutral while being operated and would not go back into gear. Transmission finally shifted to first gear, but would not change to another gear. There have been software upgrades to this transmission, but that is not considered a repair for gm. Gm says there is nothing they can do because it is not under warranty anymore, but problem started before vehicle was out of warranty. Vehicle has been taken to a dealership for concerns multiple times, but nothing was repaired. It was stated that was a common problem for that year of vehicle. Now they think the problem is pin pointed, but the repair cost would be charged to me since it is out of warranty.