Not many seems to know about him yet on my server. did him twice today and won one of them.
100% dropchance from G'nathus, whom have 43 million hp. He hits extremely hard and while he can be 5manned with 1 healer, the healer really have to be up on his game to handle the incomming tank damage. I ended having to drop heals on tank to help out, being an elemental shaman.
G'nathus seems to have about 20 mins respawn time. The spawn is a large eel pet matching the boss, and unlike other fish pets it doesnt float in a buble, but snaking through the air behind you. Probably because its much larger than other fish pets and too large for a bubble.
Edit: Also scored a sunreaver microsentry from the haywire sunreaver construct. Seems everyone whom was in the fight wins one.