All owners of Saints Row IV are getting bumped up to the Saints Row IV: Re-Elected edition, plus cross-play across different stores. So players on Steam, GOG and Epic will be able to play together.
This means everyone who didn't have it before will gain access to the Enter the Dominatrix and How the Saints save Christmas mission DLC, plus various outfits and weapon DLC (there's quite a lot) including the Anime Pack, Dubstep Remix Pack, Presidential Pack, Wild West Pack and so on.
From the announcement it will go live for Windows on December 8th, with a note that it will be "later for Linux players". The Linux port was originally done by Virtual Programming, who we've not heard from or seen ports from in a long time, so it will be interesting to see if they do fully hook up the cross-play for the Linux version too.