I’ve been hearing about this place quite a bit, and it sounds like fun. This is an instanced, storyline zone. What that means is that you and your group have the place to yourself, I think it can be raided, too. And if you leave the zone, you won’t be able to come back for several hours.
In the gully, there is a chain of quests you can do. I’ve heard rumors that it might involve digging up graves and killing the undead therein, to avoid having them all summoned up against you when you fight a boss.
There is quest you need to do to get access to Firemyst Gully, and you need to be at least level 15 to go there. The access quest begins with a dwarf at the Claymore in Antonica, and the entrance to the Gully is near the entrance to Blackburrow.