This is mainly directed at Crate team but feel free to comment.
Some time ago, maybe couple weeks GoG launched a new feature. It allows you to get a GoG key for games that you already have on Steam. For whatever reason, just as a backup, if you get your steam account banned or stolen or maybe like me you would like to get rid of steam all together. Unfortunately this feature doesnt work for all games. Earlier today i stumbled upon Grim Dawn steam on official GoG twitch channel and asked when GD will get this feature and they said that game owners have to agree to that and its not that easy. So i thought i will ask Crate if maybe after you’re done with FG you could look into that and do ppl like me a solid. Its not a big deal , just thought that maybe its not too pain in the ass to do .But again get me some FG first 
GoG has been doing this for a while now. The main goal for doing this is to encourage people to buy their games from GoG. See https://www.gog.com/news/introducing_gog_connect
You are correct that the devs need to participate.
Ah didnt know it was going on for so long - just received a mail a couple weeks ago.
Im 95% sure grim dawn has been on connect in the past. So perhaps they may offer it again.