Re: Gms?
Probably not, I don't see a point for a "GM" to be in Diablo 3.
Besides Blizzard have an official forum where you can ask questions on technical problems, as for game issues you can post them here or their GD.
Why wouldn't there be GM's? The only reason Diablo 2 didn't have one because it was in the late 90's and their resources were limited.
Blizzard is a huge company now, and the satisfaction of their costumers is top priority so you can expect a dedicated group of people who are going to be there 24/7 to assist gamers in whatever they need help with. Albeit it may not be for the reason you see in WoW, but there is the need for them.
If by some odd means their character gets stuck leaping off a ledge, or if someone is griefing extensively or exploiting a bug. What are you supposed to do then? Post it on a forum? No, you put in a 'ticket' for assistance.
The assistance may not come to you in the game like in WoW, but it may be accessible outside of the game in a Bnet channel.