Anything is possible.... it is probability that seems to get us one way or another... How long was the grinding allowed to go on? How much has the trans been operated in it's current condition of distress?
As another has commented, any diagnostic process is going to begin with "basics".... we begin with the least intrusive, simpler checks and tests and gradually work deeper - much like a doctor.
According to the WSM, the G5M-5 trans has a synchronized reverse..... Can the shift lever be moved into the reverse gear position? Are there any abnormal noises? If so, when do they occur? How many miles are on the car?