Even if they don't actively support Linux, it'd be a huge step up to add a download area for enthusiasts where you can get Linux binaries for games that have them, and set up some community forums to help people share Linux packaging/installers. Then if that's popular, they have their justification for making it more official. The idea of pushing Linux support is primarily convenience at having a simple installer (that's 90% of what GOG is for Windows users as well), but they still have access to developers to get Linux builds that wouldn't otherwise be legitimately available to people that have bought the game.

They already have some popular GOGmixes for Linux support, it'd be fairly easy for them to set up a semi-official forum for unofficial packaging of the games they sell.