Factorio: Space Age continues the player's journey after launching rockets into space. Discover new worlds with unique challenges, exploit their novel resources for advanced technological gains, and manage your fleet of interplanetary space platforms. For more information about what comes with the expansion, check out our Space Age content page.
Looking at the forum, I’m not the only one. https://www.gog.com/forum/factorio/no_downloadable_space_age_installer_for_linux
In fact, the serial key that GOG provides can be used on the official factorio website to download the game. I think this is amazing! I’m currently downloading the 4GB file right now.
Pretty sure it also works with Steam purchases btw, but you’ll have to create an account on their website and link your steam account to it