Kayato: Sorry maybe I got confused with the terms. For example:
The ability to subscribe to the Syndicate series forum to receive notification whenever someone creates a new question.
I ask this because some forums are rarely attended and a question could go unnoticed for months.
Sorry, I don't believe there is such a possibility.
At least - not in the way, you might want it to be.
You will have to click on the "community" -link in the upper or lower left corner (see pic1), to go to the games' subforums.
If you already own the game in question, its forum should be set automatically to "this is my favorite forum"and thus will show up in your "community" - section.
Else you'll have to search for the game in question.
After you visited a subforum (Syndicate), the link to it will be greyed out.
As soon as anybody comments (or opens up a new topic/question) in that subforum, it will be shown white again, with a time or date attached, to indicate, when the last change was made. (see pic2)
However - you must control that personally - GOG's forum won't alert you about a new comment/topic/question.