I checked the ColorSync Utility of my mac and it tells that gamma of 2.4 is being used.

This is normal.

But is ACAD doing some strange gamut/gamma conversion here?

I checked with Digital Colour Meter, that when I click #9, Palette shows 192,192,192, the real Modelspace background is that.
But the "New Color Swatch" shows 203/204.

Same thing happens when I try to change a color of a Layer.

#9 is listed as 192,192,192.

But now the box even looks like that shade of gray:

 The question remains: how do I change #9 to be 242,242,242?

Btw, I'm using acad2020, since the 2012 version installer has this bug, that if you run your mac from external boot drive, the installer detects that non-case-sensitive disk as case-sensitive. (I guess they won't fix this ever and I'm not buying a new mac with bigger soldered ssd in it just to run acad.)