I'm still waiting for the Linux DLC installer on GOG... It's available for Windows and Mac, but not for Linux (yet, I hope).

I agree with you! Having it on Steam, do you still need to use the Steam launcher to play games that don't have DRM?

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.