Thanks for replies, but it wasn't the door ajar or the seat belts. I just kept restarting the gladiator until the vehicle would move backward when the gear selector was placed in reverse position.
I put my 2020 Gladiator (3,500 on odometer) in Reverse, but it would not move. It would, however, move forward in Drive. After re-starting engine 5 times, it finally moved backward in Reverse. This happened to me only once. Took it to Jeep dealer service department - since they could not...