Sometimes gossip is fine. You are telling the truth--say, a worker on the job got caught stealing. Others may also be stealing and they didn't get caught yet. Gossip of this sort might just get someone to stop stealing, knowing others are watching. Which would save that person from getting in big trouble, plus prevent that person's theft. Or, gossip can be used to let others know that a person has a specific need--that's often how people figure out what to give them for Christmas.
Now, the kind of gossip that is referred to as "harmful gossip" is the kind that people say nasty things, with the viewpoint of undercutting a person's reputation. You see this kind of gossip all the time in politics--even though it might be true, it is usually out of context. This often becomes slander--which is intentionally lying about someone with the intention of ruining their reputation. That is supposed to be grounds for getting disfellowshipped when it reaches slander.
Problem is, the witlesses themselves are usually as guilty of harmful gossip and slander as any politician. Even among hounders, it is not uncommon to hear harmful gossip and slander. And they almost never get removed from the body of hounders, let alone disfellowshipped.