This elite has 43,613,700 health, and one ability which is Lightning Breath. He was hitting for about 200-300k on a tank. With a full party of five, one tank, one healer, and 2 DPS, I could see people being able to down G'nathus without too much trouble. Unless you're on a PVP server. Then you'll probably get harassed quite a bit lol.
G'nathus is affected by roots and snares, so if you can kite, I would definitely recommend doing so. Especially if you're doing it with a smaller group. The less damage your tank takes, the longer you'll be able to work on whittling down G'nathus' hp. Other than that, though, there's not much else you need to be aware of. There are a few 90 elites wandering the area, but you can get by them pretty easily without agro'ing.
I'm sure a hunter or a warlock, or any class with a pet that can tank a few hits will be able to solo this easily. It'll take a heck of a lot of time, though lol.
The key is to just stay out of melee range, as its Lightning Breath ability doesn't hit for nearly as much as a melee hit.
After about 30-45 minutes of kite dps'ing, we managed to kill this giant eel. The pet dropped as well as a blue agility trinket (forgot the item level, sorry). We didn't stick around to see it's respawn time.
Since it dropped a piece of gear, the pet could be 100% drop and the gear has a chance to drop with it. OR the gear could be 100% chance drop, while the pet drop is more random. OR both could be RNG drops. I really hope it's the former, though, since this beast took some work. And being on a PVP server, I can only imagine how much more trouble people are going to have to go through. How terrible would it feel to finally defeat this big guy and nothing drops. :(
Fortunately, for those unable or unwilling to take a shot at G'nathus, the Spawn of G'nathus is cageable. Expect to see this pet going up on the Auction House for quite a bit shortly after patch 5.2 hits. The price may go down after a while, but it'll depend on the demand.
If the respawn time isn't too terrible, I might set up a group within my guild to kill G'nathus on a regular basis. Pets for everyone! :)
Edit: Just checked back and G'nathus was there again. An hour or two, I think? So it doesn't seem like his respawn timer is too long. However, keep in mind that this is the PTR and timers are often sped up to accommodate more testing. Maybe I'll get a chance to fight and kill G'nathus again on the PTR to test the droprate and respawn time.