Hi there
I can't seem to find an FAQ for game developers, or a sub forum. is there such a thing? The only link on the main GOG website is to "Submit your game".
Any leads would be appreciated.

This isn't a game developer forum, just a storefront with a forums for the games they sell. You can submit what you have created but that's it. Depending on the tools you use as a dev there are forums for each software, e.g. Unity, and general ones. Not sure what exactly it is your after, selling or dev chat? You can also find most information under support, e.g.
https://support.gog.com/hc/en-us/categories/201552985

Thanks for your reply. Unfortunately that support section is not very useful for developers or potential game submissions. I'm looking for information such as how GOG promotes games, how a game becomes "featured", who decides on sales and reduced prices etc.

Drop support an email, to start the discussion. They will run through all your points, and I am sure some of those will be specific to certain devs which is why there isn't a set plan.

OK, will do. Thanks for the advice.