"I see. Could you fully describe the expected behaviour?"
I attached a screenshot of the keyboard layout of Greek Polytonic in macOS.
You'll see, by comparing it with the one in Trisquel (see the OP), that there
are diacritics and combinations thereof that are missing. Also, what you see
in the macOS screenshot is not everything; there are further symbols that I
can access by, for example, pressing Alt + a certain key.
So, I followed your instructions, but the only option I find is "Greek,