I was thinking of buying, but no, thanks to this thread.

Same here. I would love to buy the game GOG, but with the missing updates and all... nah, I''ll pass. I have thought better of 11 Bit Studios...

So it looks like it's still couch Co-Op only, but now that Children of Morta is playable through the Cloud on Luna Gaming maybe there's a way to get Amazon Gaming team to enable Online Co-Op ? I mean another GOG title Stardew Valley can Online Co-Op through Luna Couch.

Looks like Luna Couch is actually Online Co-Op. Here's the explanation from Luna Couch Cloud Gaming instructions:
"With Luna Couch you can play local co-op games with your friends, even if they aren’t subscribed to Luna. Just launch a game, start a Luna Couch session and share the code with your friends. Ask them to go to luna.amazon.com/couch to join your game."

So, as long as one of you or your friends has Luna+ subscription / Amazon Prime membership and a GOG copy of Children of Morta, you can online game it together through the Luna Couch feature. Not as ideal as doing it the Steam way, obviously but if you already bought Children of Morta on GOG you can at least sign up for a free Amazon Prime membership and knock it out with a few buddies for those 30 trial days. As well as whatever other titles you may have amongst you that also support Luna Couch.

To be honest I looked into how the online coop of Children of Morta on Steam works and I´m not surprised its not on GOG yet...It´s friends only...Propably via list...Is joining via friends list on GOG Galaxy even a thing? As long as that's not a thing, I highly doubt it will ever come because I´m sure they won't make a completely seperate online coop system just for GOG.