Funnily I actually stumbled upon this thread while checking up on whether there's been any new posts about
Showgunners which we just released a week ago. :)
We actually have always had some presence on the subforums for our games (Hard West, Phantom Doctrine and now Showgunners) but I must admit, it's probably mostly because I was a GOGer before I was a game developer and I have always also urged colleagues to take GOG seriously.
(side note: our presence on our past games' subforums stopped when we lost ownership of those and literally couldn't support them anymore)
Anyway, Steam is far more significant than GOG in pretty much every way: Sales are higher, a high user rating on Steam is one of the most significant drivers for sales, pretty much every game developer is personally familiar with it already and so on. Importantly, there is also much more user activity on the Steam communities than on the GOG subforums. For comparison: on Steam we have over 150 threads about Showgunners already while GOG has... 2... both of which were created by the same user. On Steam there's basically an ongoing stream of feedback while on GOG it can sometimes take days, weeks or even months for anything to appear.
It is comprehensible that with such proportions most developers figure that they should just focus their efforts entirely on the Steam community. It is also relatively unlikely that on GOG a user will report anything significant that has not been already reported on the Steam community - and fixes based on Steam community feedback will also make their way to the GOG version unless a developer has simply given up on supporting the GOG version altogether (which I personally consider unforgivable).
And I really don't want to take a dump on GOG itself, I love this place for many reasons but of course there's also the matter of convenience for developers themselves. GOG has been gradually improving developer tools and support but it is still a far cry from the competition and I hope GOG will keep improving these things for the developers' sake, the users' sake and ultimately also their own sake.
Mind you that I don't condone ignoring the GOG community this way at all but I think that's most of the reasons why developers aren't generally active here. GOG is peculiar place with a peculiar culture, a peculiar community and also peculiar demands. I can see how it's difficult to even understand GOG and its community if one wasn't a user of it first - and even becoming a GOG user is a choice that requires a peculiar mindset. :)