Looks like downvoting abuse has been running rampant recently:

https://www.gog.com/forum/general/gogfix_the_rep_or_delete_my_account_now/page1
https://www.gog.com/forum/general/alright_gog_time_to_end_this_shit
https://www.gog.com/forum/general/mahayo_redux_gog_fix_your_forum/page1
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Here are my suggestions on how to fix it, with no need for appointing moderators.

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Why no dedicated moderators?

It's obvious that GOG wouldn't have the resources to properly moderate the forums. Appointing well-known forum users as moderators could cause a lot of damage to the community if one of them decided to abuse moderation privileges.

Voting-based moderation (when implemented properly) also puts the power in the community's hands rather than a few appointed people.
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What needs to be done to prevent abusive downvoting?

==Add a reputation cost to downvoting==
One forum I know has a downvote option where if you downvote some types of posts, you also lose reputation along with whoever you downvoted.

The GOG forums would benefit from this - people who make trash alts or abuse the downvote button will see their reputation take a dive.

==One reputation change per person per thread==
Currently, if some lowlife is running a downvote campaign against you, they can downvote you once per post. This just results in a toxic forum environment where people can potentially be punished for each post they make.

The solution:
Each person who downvotes/upvotes your posts in a thread can only affect your reputation once in that entire thread.
For example, if I post a thread,and then later post 2 replies (3 posts total), and then some lowlife downvotes all of them for no reason, then I only lose reputation equivalent to one downvote. If I post even more in that thread and the lowlife keeps downvoting my posts on site, I wouldn't lose any more reputation.

A similar rule would apply to upvoting: if you upvote every post that a user makes in one thread, the reputation effect would only count as one upvote.

The reputation change limits would only affect actual reputation; each vote would count normally toward a post being marked as high rated or low rated.

==No more silent votes; require that people post in a thread before upvoting/downvoting anyone==

For the most part, silent downvotes aren't useful, except to prevent threads that are obviously spam. If someone's posting crap, it helps to explain why the downvoted post is crap.

Obvious solution: "MAHAYO" (Make A Human Answer You). Hide/disable the reputation buttons until the user has posted in the thread.
Even better, if a bunch of trash alts want to mass downvote someone, they now each have to post in the thread (time-consuming for lowlifes who abusively downvote) The community would now be able to see the trash alts and also have the chance to hammer trash alts with downvotes.

==Make all upvotes and downvotes public==

People need to be responsible for their conduct. To prevent abusive trash alt downvoting, I recommend adding an option to every post where you can view a list of upvoters and downvoters. That way, the community can easily figure out which ones are lowlifes/trash alts.

==Add a weighting/leveling system based on how many games a user owns or has purchased==

Trash alts tend to have few or no games (other than the free ones) registered to them. On the other hand, most legitimate forum users will have a few purchases or at the very least some games gifted to them.

Suggestion: Have a "leveling" system where your level is based on the total cost of games you have purchased and the number of gifts you have received:
-Buying a game (no matter if you bought it for yourself or gifted it) gives full contribution to your level
-Redeeming a gift gives half contribution to your level.

Why does this work? Lowlife scum aren't likely to register a lot of games to all of their trash alts, and known scum/scammers are usually called out in the "list of known scammers" thread and excluded from giveaways.

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Putting it all together:

Take a look at this discussion. The OP is having every post mass downvoted on sight (obviously unfair)
https://www.gog.com/forum/general/gogfix_the_rep_or_delete_my_account_now/page1

If these suggestions were implemented, things would be much different:
-The OP's reputation wouldn't be dropping anywhere near as much because of the "one reputation change per user per thread" rule, preventing people from being unfairly downvoted for each post in the thread.
-There would be a visible list of trash alts downvoting the OP
-The trash alts would have to actually post in the thread, giving the community the chance to downvote them right back.
-The trash alts would quickly lose reputation, as their downvotes would cost them as well as the rest of the community downvoting them.
-Trash alt downvotes wouldn't count for much due to the leveling/weighting system based on total value of purchases and gifts received

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Let me know what you think of this suggestion. Counter-arguments, constructive criticism, support, upvotes, etc. are all welcome.

Also, if you see lowlifes/trash alts downvoting this thread, PLEASE upvote it - prevent them from hiding constructive posts like this.

How on earth will this prevent downrepping by alts (which is what is happening now)? How on earth will this prevent downrepping by alts (which is what is happening now)? How on earth will this prevent downrepping by alts (which is what is happening now)? How on earth will this prevent downrepping by alts (which is what is happening now)?

To illustrate my point I created an alt account.

This is the same guy who posted that the government was trying to kill us by putting fluoride in the water. I don't think logic is his strong suit. :P

This is rule by the masses, and as we have seen, rule by the masses on the internet DOES NOT WORK!

His arguments seemed perfectly legit to me, a system like that doesn't do enough.

I think that was JCD-Bionicman. Yup.

"This is rule by the masses, and as we have seen, rule by the masses on the internet DOES NOT WORK if there are no consequences for abusing the vote and no way to contest/counter against unfair votes!"
FTFY

The main problem is that a plain unrestricted up/down vote system turns into a mob-ocracy. My suggestion preserves the voting-based moderation, while adding restrictions that protect honest, well-behaving users and obstruct abusive users/trash alts.

Or you could have a rep minimum for voting like you do for posting links, but make it higher, like maybe 100. It takes a while on the forums to get a 100 rep and would be quite a bit of work to create a legion of alts with at least 100 rep in order to screw with the post voting...

That's what I said too in another thread, seems to be a logical quick to implement solution to me.

But this just moves power from one base to another. Like the rich getting richer thing. Not something I would agree with.
DivisionByZero seems to have some good points.

Not really because those who have over say 100 or even 200 is going to be much more limited than those who have 0, which is everyone. The biggest issue is trolls, this would essentially eliminate the alt downvoting because who is going to take the time to get that many alts that much rep.

The problem, as others have defined it, are users with zero or negative rep creating innumerable alt accounts (also with little to no rep) in order to abuse the voting/rep system. The system as proposed by the TS would not stop such a thing. Earning 100 rep takes some time and work, hopefully too much to make creating a ton of alt accounts with enough rep to make a difference voting-wise worthwhile.

The problem actually seems to be that the power base is even. New users have too much power to affect long time users or even just anyone. If new users can't post a link, they also shouldn't be able to vote.

But once again, a shift in power, and with power come corruption. Get enough of the regulars together who don't like a person and that person is then bullied into silence. This can go either way. At least with the system we have now it is completely open and free, with no community players being involved. It's not perfect or even great, but at least its honest.

But they already have that power. You're merely taking away the power from those that aren't at least a little established.