I bought a GM1 (identical in most respects to the GM5) prior to buying the Sony RX1R II. The only thing that is almost identical between the GM1/GM5 and the RX1R II is the battery life.
The shutter on the RX1R II is quieter. The low light performance (AF, IQ, noise, etc) is without match as is the boket via a 35mm (17mm) lens.
I read that the output from GM1/5 is 12bit and that drops to 10 while using adapted lenses and/or the silent shutter.
Apparently so, yes. Fortunately at night the GM5 shutter is very quiet (from what I see quite than the GM1's I remember), and I am pretty covered in speeds from 1sec to 1/500. I am normally shooting at 1/50, 1/30, 1/125-1/250 depending on subject & lens, so I don't have to deal with that.
In daylight the drops affects it much less because you are shooting at a lower ISO unless you are in a very high contrast scene I suppose, but so far haven't bumped into particular issues.
Despite all this, I would have bought a GM5 if it only had a tilt/swivel LCD. The fixed LCD was a deal breaker (not interested in the GX7/8 due to size). Perhaps the GM7 would offer it together with a better shutter.
I am expecting about the same shutter with the GM7, but I am also expecting the new GX8 sensor with slightly better chroma noise and color sensitivity.
From the Autofocus I have seen, I am willing to bet the GM5 focuses faster still than the R1XRMKII. The AF of the GM5 is crazy good- pretty surprising when I see it focusing the Olympus 75mm F1.8 prime in CDAF and better than the OMD EM5 MKII itself.
The RX1RII can't use faster lenses, some of those close some of the big gap between the two at night. My main issue is simply I am not a 35mm shooter and if I have to crop constantly at that point using a proper lens on M43rds would match in many situations for the most part or come closer.
Also the fact the GM5 is still smaller and I can put it in a pocket with its kit lens or even some of the other primes... can't do that with the RX1RII. To me that's a huge diff too.
Not to say the RX1RII is bad, it's pretty phenomenal obviously. But the GM5 is the Pentax Q I wanted. And to my surprise, turbo charged.