I wondered if we can also get a GOG-Key instead of a Steam Key. I did some search and I came across this

by Kenneth Stephens:
" Just a heads up for anyone who doesn't use steam, but wanted to play HLD on their PC using a drm free humble bundle or gog version: you can't use the alt drifter. Fortunately Heart Machine responded to my email and agreed to refund me, so if anyone else is in the same situation as me, then you should email them."

https://www.kickstarter.com/projects/1661802484/hyper-light-drifter/comments
search for "gog"