But I wonder if Heroic can install HoMM2 on a Mac if no Mac binary is available on GOG.

I have to admit, I've tried using Heroic Launcher both on macOS and on a Steam Deck and in both cases the experience has been rather frustrating. Maybe I was just unlucky, but any game I tried was broken in a way or another, or stopped working with no clear explanation (weird segfaults at start) just a few days later.

I've tried GOG DOS Games, GOG Windows Games, Steam Games under Proton, nothing was particularly usable.

Got the very same games working under Faugus Launcher and Dosbox-X. The UX is a bit worse but my games run without any issues, and I've stuck with those since then.

Heroic Launcher is awesome on the Steam Deck