Recently, GOG added the Windows version of XCOM: Enemy Unknown to their store and since it has a Linux version on Steam, I reached out to the porter to see about their plans for the Linux version.
We've seen a lot of speculation in the past, with people wondering if Feral Interactive will ever get their Linux ports onto a store other than Steam. Here's their official stance, which they sent me this morning:
We don't have any plans to distribute our games through GoG. If this changes, we'll make announcements through our usual channels.
We can speculate all we like as to why they're not doing it, even if the decision does strike me as a little odd. Hopefully they will reevaluate this stance in future, considering it's not exactly a new game and the Linux port from 2014 isn't exactly new either.
A shame for everyone who prefers their games on GOG.