The Gate Spell is a type of Null magic that allows the user to travel to almost any previously seen location.
Basically, the Gate Spell is a form of teleportation, allowing the user to travel great distances in an instant. Once invoked, the spell will open up a circular portal, allowing the user and anyone else he wishes to pass through it and appear at the destination. There seems to be no limit as to how far the user can travel. Apparently it can even travel to other planes of existence, as Touya was able to use it to visit God.
Gate will only allow the user to travel to places he/she has been to previously. By itself, it cannot allow the user to travel to unexplored or unknown regions. However, if used in conjunction with the Recall Spell, then it would be possible for the user to go to a place he/she has never been to before. Additionally, since the user must physically walk through the gate, it cannot be used if the user is paralyzed.