if you followed the guide and created the rclone@.service service file, you should be able to start the service like
systemctl --user start rclone@gdrive
To have the service running on boot, it would need to be enabled. This will create the sym link automatically
systemctl --user enable rclone@gdrive
Syncthing is executed as user dietpi. Therefore you would need to allow r/w access to this user to /mnt/gdrive/