gh follows normal conventions regarding exit codes.
If a command completes successfully, the exit code will be 0
If a command fails for any reason, the exit code will be 1
If a command is running but gets cancelled, the exit code will be 2
If a command requires authentication, the exit code will be 4
NOTE: It is possible that a particular command may have more exit codes, so it is a good practice to check documentation for the command if you are relying on exit codes to control some behavior.