GM-Island is small island below PVP-Island
. It has no monsters, stores, portals or harbors. In this island You can find small house with some tables and "prisons" inside of it. The only possible way to get to GM-Island is to be teleported there by a GM. In earlier days of TibiaME, there wasn't so many players, so GMs were interacting with players more often. Some lucky players got to see the island. Here's a video of Ryomaonii and Raistlin
chatting with GM-Isolan and exploring GM-Island.