@majorc 0.0 gain or 0.0 db means the DAC is connected straight to your pre-amp or power amp and is not interrupting or changing the voltage from your source.
-db means the gain has been reduced and +db the gain has increased which will decrease or increase the volume by a such a multiple.
Your Ifi DAC probably has a digital volume control but don't sweat it. Only in a more revealing system would it show wanting. It probably sounds fine.