I've uploaded four pictures showing where the changelog is mostly accessed. I don't think too many people check the actual game forum, unless they have issues or there are general questions.
1 and 2 are via the game list in your account. That is done in the browser but can probably also be done in the client (never checked that to be honest, my browser is faster).
3 and 4 are in Galaxy and you can click on the "1.3 -> 1.31" for the changelog to open.
No idea how to do it from a developer's perspective though. You would have to look at it yourself (or ask some GOG staff, I guess). And by the way, you should get yourself developer recognition, so that your text appears yellow-/orangish.
Oh, thank you! I will have to find out how to fill those info. At this moment, looks like I haven't any access to edit or add any changelog information. Maybe it's because I'm new in GoG, so, I will contact to developers support to ask about it.
Thank you to explain how users access to changelog here =).