It's hard to judge exactly how many players know about GM Island. Some of the older players will remember the flood of trade chat and guild chats whenever a GM was online and visible in GM Island, and those folks who like to mess around with private servers and unexplored places will have at least heard of its existence. However today, with the island being so cordoned off, many players probably are not familiar with the mythical island of the game masters.
GM Island is a piece of land that used to exist in the very upper left hand coordinates of Azeroth, far beyond the reach of players. It exists as an in-game entrance point for Blizzard GMs, and is a relatively mundane place. It has a small keep and a dock, a few trees and bushes, and that's it.
There was a time when players were able to swim out to the island, and that swim took around an hour of hard work. It involved using a level one character constantly casting renew and other healing spells; and the player had to know exactly where to point themselves or risk being hopelessly lost in the vast oceans of Azeroth. The trick used to get there was similar to the one that allowed players to get to the Isle of Quel'Danas on the patch 2.4 PTR back in 2008. It was also possible to water-walk out there, but not much is known anymore about that method, at least publicly.
After the break we'll take a look at a few of the features and lore surrounding GM Island, as well as look at some videos. However, please be forewarned that some of the images and videos in this post are from private servers. Enough legitimate in-game footage simply doesn't exist.
It is absolutely impossible to access today via normal means such as swimming out to the island or by using an in-game exploit. If you try hacking the client to get there, chances are the warden won't let you log in.
The moral of the story here is that you shouldn't even try to go to the GM Island. It's not worth the risk and you're just asking to have your account entirely revoked for even trying to get there.
Obviously when Cataclysm hits players will attempt to fly out there, and many will no doubt come up with ingenious ways to make it across the vast ocean and avoid fatigue for as long as possible. However, the island now exists inside an instance and cannot be accessed via normal means.
I don't want to talk about my own experience on GM Isle too much, since the Blizzard employee nice enough to send me there obviously values his or her job; but it's a relatively empty place with nothing special there. No mobs, no NPCs, nothing. Just a building as seen in the pictures. There's also no portals, vendors, herbs, mining nodes, etc. All of those items were part of Developer's Island, which is another place entirely.
However, one very interesting feature exists...
The Player Jail, a.k.a. The Box
Back in the day, around the time of the game's launch, players could be transported into a timeout room where they'd have to wait to talk to a GM. The room resembled a Star Trek Holodeck -- it was pure white with blue lines running in block patterns around the entire three-dimensional area. There were no entrances or exits, and players could only leave by hearthing out; which the GM could disable by removing their hearth stone.
There was, at one point, a single chair upon which players could sit on. That chair has long since been removed. A WoWWiki edit indicates that the chair has returned in patch 3.3.3, however we've been unable to verify this claim.
There are two images that I could find of the player jail, and it's a relatively mundane area.
Blizzard has publicly acknowledged the existence of GM Island before on a few occasions. Recently, Zarhym said the following when prompted about its current use:
Is GM island real? And if so, what the heck was the point of making it?
Yes. The game masters who respond to you in the game are doing so through a separate chat/support tool. They're not actually using the game client to whisper you, however, logging into the chat client means the character they use to contact players with is logged into all of the realms they need to. While GM invisibility exists, it makes it safer and easier to have a simple storage/port point to keep all of these characters out of the normal game world.
There was, at one point, some lore associated with GM Island in the two characters Ari & Tuskfyre. WoWWiki says on the subject:
Strangely, Blizzard does mention GM Island in some lore. In their recruitment section, on their official European website, the company explains how players from every race (Horde and Alliance alike) do their best to make sure any technical problems in Azeroth are resolved. The gnome "Ari" and the troll "Tuskfyre" are two of these inhabitants of GM Island. These characters are nowhere to be found elsewhere in-game, however.
Videos of GM Island
Below are a few videos of GM Island. Most are from live servers, however from several years ago. Remember that it's now impossible to get to GM Island by normal means.