There is zero evidence that this is the case. I guarantee the vast majority of GOG users have never heard of Devotion.

GOG also do advertise. They've been reaching out to hardcore users for feedback, they have rolled out campaigns to promote their Capcom old game licensing and their new game request system and they've implemented an ad tab in their launcher (that they keep sending people polls about for some reason).

Much more likely what is happening is they are being choked out by Steam and DRM. With large publishers increasingly wanting data streams from their games, AAA releases on GOG are less justifiable. Sony pulled out when they started adding a PSN login to their games. It's harder to tell Sega's reasons, but they may have just been a desire to keep their increasingly popular Ryu Ga Gotoku stuff DRMd for longer. Indies, meanwhile, are cash strapped and MUST default to Steam, with support for any other platform being an increasingly unjustifiable spend. With Steam banning competing on prices (beyond free giveaways and subscriptions, it seems), there is just no competing.

So no, the DRM-free hook alone isn't working for GOG. Working here would mean growing or at least preserving their stake against Steam's all-consuming rage, and that's clearly not happening.

Which is why I'd love for people to start acknowledging the issues with Steam and their position and start defaulting to GOG where they can.