Go to the Full Parameter List, click the Value field for that parameter and you will see it’s a bitmask of selected constellations. Choose what will work best for your part of the world. 65 (GPS&GLONASS) will work well in many parts including some on here from Australia that make that recommendation as I recall.
Do a few tests and see what works best where you are. Look for reliable and consistent HDOP. Also check the GPS Delta for the update rate, it should be very consistent around 200ms.
So depending on your update rate and reliability of HDOP you could select more or less or different combination of constellations.
Usually people outside the USA would select GPS plus one other constellation, and in the the USA you could also add SBAS. These days Galileo could be a better choice that GLONASS.
The issue is that a GNSS unit can get bogged down trying to use too many constellations and the update rate suffers. Also I’ve seen where additional constellations must affect some internal calcs and the HDOP can even suffer. (I’m being very nonscientific here, but you get the idea)
For example where I am there are plenty of Beidou sats visible, but if I add that into the mix the HDOP is higher and it takes longer to acquire a good 3D Fix.
I think GPS_GNSS_MODE 0 is the default because the fallback is the capabilities of the physical GNSS unit. A while back they could usually only do 2 constellations and ignore anything else. Since there’s been firmware updates and constellations have expanded, I think we need to manually select this parameter to suit our own situation.