Site says

"Want to win a copy? Simply say "I love DRM free games!" somewhere in your comment to be entered. Competition closes 7PM UTC tomorrow, winners will be sent a PM directly here with a key if they win sometime after that."

https://www.gamingonlinux.com/articles/weve-teamed-up-with-gog-for-the-ubuntu-1804-release-we-have-some-keys-to-give-away.11661