At the moment people that backed this game, DO NOT receive a GOG key to redeem the game.
I am quite disappointed by that fact. I hope they find a solution. But communicatons from the devs on this topic is ZERO.
After all this hazzle with the backer rewards and the GOG key I would think thrice to give them any more of my hard earned money.

Considering they sold (probably in terms of delivered) one million copies of the game and now there are 100,000 people playing the game on steam simultaneously, they have other priorities like patching the game, which was published too early.

In the forum of the game (https://forum.kingdomcomerpg.com/t/backer-copies-only-redeemable-on-steam/36318) there are only ~ 50 backers complaining about not getting a key for gog, but steam instead. This makes it easy for them to brush the request off.

The way they handled the manual, the poster, and the metal box indicates nothing good for the gog key request. In yesterdays update they explained why they didn't deliver those items. If they (full of embarrassment) wrote this ten days before the release (not after), people would still be sad.

The support wrote: "[...] all the contracts, database, taxes and so on are connected to Steam. To change this whould [sic] need a huge amount of effort, which we can´t do, because we don´t have the capacities to do so." I would pay an additional (DRM-riddance) fee to convert my steam key to gog key!

I hope in the wake of the release at gog.com they might think about their backers there.

Dunno about gog keys, they would be nice, but at least in early stages of development they did not commit to drm free release and it was planned as far as I know to be Steam only. Which is why I did not back the game and bought it here.