“Griefing” is not anything actionable. It’s a very vague term that the player base uses when they don’t like something that someone else does.
PvP combat is allowed on PvP realms. It is even allowed on Normal realms, but it’s simply that the other player must turn on Warmode, or do some other PvP action (PvP quest, run through an enemy city, etc).
Ganking can become harassment if it’s documented for days, even weeks at a time. But there is a lot more that goes into it, including not allowing a player to progress the character and/or happening across multiple characters, just to name a few off the top of my head.
So basically, have at it.