Casting a gate spell has two effects. First, it creates an interdimensional connection between your plane of existence and a plane you specify, allowing travel between those two planes in either direction.

Second, you may then call a particular individual or kind of being through the gate.

The gate itself is a circular hoop or disk from 5 to 20 feet in diameter (caster's choice) oriented in the direction you desire when it comes into existence (typically vertical and facing you). It is a two-dimensional window looking into the plane you specified when casting the spell, and anyone or anything that moves through is shunted instantly to the other side.

A gate has a front and a back. Creatures moving through the gate from the front are transported to the other plane; creatures moving through it from the back are not.

Planar Travel: As a mode of planar travel, a gate spell functions much like a Plane Shift spell, except that the gate opens precisely at the point you desire (a Creation effect). Deities and other beings who rule a planar realm can prevent a gate from opening in their presence or personal demesnes if they so desire. Travelers need not join hands with you—anyone who chooses to step through the portal is transported. A gate cannot be opened to another point on the same plane; the spell works only for interplanar travel.

You may hold the gate open only for a brief time (no more than 1 round per caster level), and you must concentrate on doing so, or else the interplanar connection is severed.

Calling Creatures: The second effect of the gate spell is to call an Extraplanar Creature to your aid (a calling effect). By naming a particular being or kind of being as you cast the spell, you cause the gate to open in the immediate vicinity of the desired Creature and pull the subject through, willing or unwilling. Deities and unique beings are under no compulsion to come through the gate, although they may choose to do so of their own accord. This use of the spell creates a gate that remains open just long enough to Transport the called creatures. This use of the spell has a material cost of 10,000 gp in rare incense and offerings. This cost is in addition to any cost that must be paid to the called creatures.

If you choose to call a kind of Creature instead of a known individual, you may call either a single Creature or several creatures. In either case, their total HD cannot exceed twice your Caster Level. In the case of a single Creature, you can control it if its HD does not exceed your Caster Level. A Creature with more HD than your Caster Level can't be controlled. Deities and unique beings cannot be controlled in any event. An uncontrolled being acts as it pleases, making the calling of such creatures rather dangerous. An uncontrolled being may return to its home plane at any time.

If you choose to exact a longer or more involved form of service from a called Creature, you must offer some fair trade in return for that service. The service exacted must be reasonable with respect to the promised favor or reward. For instance, for a task taking up to 1 minute per caster level: a payment of 100 gp per HD of the Creature called; a task taking up to 1 hour per caster level: 500 gp per HD; a long-term task, one requiring up to 1 day per caster level: 1,000 gp per HD. Some creatures may want their payment in “livestock” rather than in coin, which could involve complications. Immediately upon completion of the service, the being is transported to your vicinity, and you must then and there turn over the promised reward. After this is done, the Creature is instantly freed to return to its own plane.

Failure to fulfill the promise to the letter results in your being subjected to service by the Creature or by its liege and master, at the very least. At worst, the Creature or its kin may attack you.

Note: When you use a calling spell such as gate to call an air, Chaotic, earth, evil, fire, good, Lawful, or water Creature, it becomes a spell of that type.