Gating

Gating is a slang term that refers to cards that require you to return a permanent (usually a creature) that shares a color with it to your hand when it comes into play, usually in exchange for a lower casting cost or higher power. Gating cards were featured in Planeshift.

Description
In science fiction, “gating” is a term often used to mean teleportation. This is supposedly where the slang comes from. All gating cards printed are multicolored. There are 13 gating cards in Planeshift, 12 of which are creatures. Gating is primary used in white but all the other colors can have access to it when needed.

The most powerful of these is the Shivan Wurm, a 7/7 trampler which could be cast on the third turn in a green deck with mana-producing creatures. Meanwhile, Fleetfoot Panther's gating cost can actually be a boon, as its flash ability can save a creature already in play from destruction. Any gating creature can also be useful in a deck full of creatures with comes-into-play abilities (like Flametongue Kavu, for instance), allowing you to return such creatures to your hand so you can play them multiple times.

As the gating cards are multicolored, the strongest color combination of the gating cards is green/red. This combination has the most gating cards with four. This combination has two of the strongest gating creatures in Horned Kavu a quick 3/4 creature for best played as a second-turn drop. Shivan Wurm can be a solid fourth of fifth-turn drop.

One should also take note of Cavern Harpy which enabled the Aluren combo.

Not a keyword
Gating is not an ability or a keyword. If it came back one day, R&D most likely would keyword it.

Visual cue
In the art of the cards, gating creatures appear to pass through a portal.

Similar cards
Shrieking Drake in Mirage had a similar ability, though the creature could be any color. Since it costs only and can return itself, it can be employed in many combos. To an extent, any creature with proper gating can too, but their higher, multicolored costs make them less efficient for such purposes, Cavern Harpy in the Aluren combo being a notable exception.

The mechanic returned slightly altered on white cards in Planar Chaos. No color is prescribed as to which creature needs to be returned but instead multiple creatures would be needed to be returned on some cards. These cards are Whitemane Lion, Stonecloaker, Stormfront Riders and Dust Elemental</c>.

The mechanic returned again on the card Kor Skyfisher</c> in Zendikar, but with the difference that the returned card does not have to be a creature, but can be any permanent you control.

Rulings

 * Gating cards read "Whenever [this card] comes into play, return a [color A] or [color B] card to your hand." That's a triggered ability, so players can respond to it going on the stack by playing instants or activated abilities.
 * Like any comes-into-play triggered ability, the gating ability will trigger regardless of whether the card comes into play from a player's hand, graveyard, or library (or any other zone).
 * The gating triggered ability doesn't target anything, so it's perfectly fine to return an untargetable card you control (or a card with the right kind of protection) to its owner's hand.
 * When the gating ability resolves, if you control no appropriate cards of the specified colors other than the gating card, you'll have to return the gating card itself to your hand. If the gating card turns out to be an invalid choice (for example, if you change the gating card's color before its comes-into-play ability resolves), then the gating ability does nothing.