It's technically open, but it contains no information to clarify what to find in the Garden of Secrets, or its uses. All the information will be made public in a few days, though I can tell you what to expect. The GoS serves as an IC Area outside the tournament itself, where the participants may interact in whichever way they decide. Moreover, as the Garden is full of dimensional rifts, you will not be confined to the Garden itself and will be able to access any setting you desire. Lastly, you may also write about events prior to your character's arrival in the GoS.
Basically, as long as you write with the character you registered and the thread has a slight connection to the ToC, you're good to go.
As for the Non-ToC related roleplaying, you may do so in the Side-Events Forum.