Updating the DAB+G is necessary, because by this you can ensure that it is recognized by your Gateway device properly.
Updating the DAB+G separately is not possible. If you want to update the firmware of the DAB+G then you need to install the update of the Gateway device. This is because it also contains the update of the DAB+G. You can find and download the latest firmware update package for the Gateway Pro BT or 500S BT from the bottom of these websites:
The steps for the firmware update process are included.