Is it always of grave matter to gossip about another’s grave faults? What if the motive is something other than to deliberately ruin their reputation and you reason that it will most likely not?
There can be reasons to discuss another person’s faults. Just reasons would make such a discussion not sinful (different to gossip).
But to gossip (discuss another faults without just reason) is often serious. Always? It’s hard to say always, but generally it is a serious (grave) matter.
Again, as with your other questions, you can get the best guidance from your confessor or spiritual director.
There can be reasons to reveal the faults or failings of someone -even serious ones - to someone else that does know it. Sure. An objectively valid reason to make the matter known to them.
Like someone is stealing from the company (one could talk to the boss). Someone is a real danger (you may need to warn someone). A child is doing something (the parent can be told). etc.