If you are certified in transmissions, fixing the reverse in a 1990 GMC S15 should not be overly difficult. The issue could potentially be caused by a number of things such as a broken or worn reverse band, a malfunctioning solenoid, or damaged gears. To diagnose the issue, you will likely need to remove the transmission and disassemble it to inspect the components in question. Once the issue has been identified, it will then be a matter of replacing the faulty part or parts. How long the repair takes will depend on the extent of the damage and how easily you are able to access and remove the transmission from the vehicle.