Is there a GOG.com article on how the reputatuion system works?
I don't know whether there is still an article on that. But the short answer is: it doesn't. Rep is thoroughly broken and all members, who stick around for a while, realize sooner or later that it doesn't mean anything.
-you gain 1 rep for each day of forum activity
-you gain 5 rep for providing the answer to a thread marked as question, if the thread opener marks your answer as the solution of the problem.
-you gain up to 5 rep for a post of you being upvoted by sufficient people (little green +)
-you lose up to 5 rep for a one of your posts being downvoted by several people (litte red -)
And that's where the 'broken' stuff comes in. GOG doesn't check whether the accounts that do the up or downvoting are actual active community members or just bots. So there are some people around here who have several alt-accounts and run scripts that seek out every single post of some person they don't like, downvoting each of them. In that way the target of such a bot-attack can lose rep very quickly. The same also works the other way round. I.e. the people using these alt-accounts can, of course, also upvote their own main account. So all in all the Rep number has become arbitrary and just oscillates up and down, depending on whether you caught the attention of one of these trolls or not.