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Focusing only on the GPS vs GLONASS part, there are a few things to note:

1. A GNSS receiver usually do output its calculated position with all additional information in the NMEA format which is agnostic of the satellite system used.

2. The entire position with all additional information is calculated on the GNSS receiver, not in gpsd. The gpsd service does forward this to any application requesting this information.

3. gpsd_client is 'just' a client program of gpsd. It will happily convert anything to a ros message whatever gpsd tells it to.

4. What I've read so far many receivers will always use only one of gps, glonass, galileo, ... whatever is currently available and not any of them combined with some sort of priority. They try GPS first, if GPS does not work they try GLONASS and so on. So you should check the documentation of your GNSSS module if it does support multiple systems at the same time.

So any value in the /fix message from gpsd_client comes more or less directly from your GNSS receiver. It is the decision of the receiver if GLONASS is used. If in doubt about values like satellites visible compare the value with a known gpsd client program like cgps.