The Gate Spell is a type of Null magic that allows the user to travel to almost any previously seen location. It becomes Touya Mochizuki's signature spell along with [Slip]. In the future, Touya first daughter, Yakumo Mochizuki would inherit this magic.
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.
Offendedly, the Gate Spell can be open directly beneath someone like a pitfall. This can range from single person to even a whole army, depending on the users magic levels. Where the user drops the victim depends on where the user can reach. Touya has used it to drop his opponent from the sky and to send army's back to their country. As well, the user can open a portal above their target so they can drop something onto them.
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; though this can be circumvented by opening the [Gate] directly under the target (akin to a trapdoor) as Touya has opened a [Gate] midair for Yae when they took on the mithril golem.