[nikdaheratik]

I think you're ignoring the fact that the person who is doing the gossiping can also be the subject of gossip. If you gain a reputation for spreading untrue negative assessments about others, then you could end up being the subject of ostracism as that behavior also can lead to results that are harmful for the group.

Which, of course, can lead to even more complex "gaming" of the system, but only to the point where the amount of effort you put in outweighs the minimum acceptable level of effort you could otherwise put into the work itself.