I got an answer from GOG support on this topic :
"We only offer a Windows version of Master of Magic because we do not have rights to distribute other versions."
So that explains it.
The answer is clear, and btw, support is nice, it also includes external links explaining how to make it work manually (i.e. without Gog Galaxy).
But it's also a bit worrying, for the GOG business as a whole, and is shaking up my trust in the whole system.
As you know, there is no Windows nor Mac version of MoM.
There's only one DOS version, which must be played on modern platforms through emulators.
So now, the right holders don't just authorise distribution of the DOS binary, they also give themselves the right to authorise _running the binary_ on a limited set of platforms, including a limited set of emulators.
I'm not sure if they really have this right, but as usual, the right holder nonetheless takes it first, and now, if it's confirmed either by the market or by jurists, it's a pretty bad precedent, for _any_ old game.
The whole promise of GOG is that you buy a license for an old game that used to run on old platforms that do no longer exist, and then run it on whichever modern platform you have, modulo technical limitations. But now, you don't buy a right to play MoM, you only get a right to "run MoM on a limited set of modern platforms that's controlled by the right holder". The expectation here is that right holders will use this power to require more money for additional platforms.
Besides, in the case of MoM, this list of platforms is restricted after being bought !
As you know, a Mac installer was available when we bought our MoM license.
Which means, the right to play MoM on Mac has been _removed after it was bought_.
So now, what's next ? Which other game are we no longer going to be able to play on Mac, or on Linux, because the license is suddenly restricted after being bought ? That sounds nasty.
As I said, this is shaking my trust in the whole edifice.
Sure, I know it's possible to download the Windows version, extract the original DOS package, install an emulator ourselves, and do the whole complex setup thing by ourselves. But that's not the point. I could already do it before (I already have a physical copy of MoM), and the only reason to buy it on Gog is to get Cloud and Installation convenience.
That is, if we can play the game at all...