There are a few keystone games that define genres. When Doom came out, it was a trendsetter that everyone was talking about and comparing others to (eg "Doom-clone"). When Halo came out, once again, it was the game everyone was comparing others to. It pioneered many mechanics that have become standard: vehicles, two-weapon limit, quick press of a dedicated button for grenade, and likewise for melee. For its importance to gaming history, it really should be on GOG. It wasn't perfect, but that is likely because it was trying so many new things. I would definitely snap it up if it came on GOG. I previously had no hope that Microsoft or Blizzard would ever put anything on GOG because they already had their own digital storefronts, but the recent Diablo release proved me wrong.
Personally, I would predict that they will release Halo MCC only on Steam at first, and some time later release the original Halo Combat Evolved (not MCC) on GOG for the "purists" who want the original, pristine, retro version. And then much later, they might put MCC on GOG. Maybe. Eventually.
But that is just my opinion.
I'll wait and see. I won't be buying it on Steam anytime soon. Maybe if enough time passes that I conclude they won't bring any Halo to GOG.