Gating

Gating is a level design technique that consists in confining the player to a small area and/or blocking the access to another area, until the player completes a given objective or makes a specific action. There are several kinds of gating, used in different situations.

Simple gating

According to the definition, a lot of things can be considered gating:

• Locked doors of which you must find the key.
• Platforming sequence to reach the exit.
• Puzzles.
• etc.

They all have a simple mission: extend the lifespan of the game, and bring some variety to the low-level gameplay loop.

Region gating

The player is barred access to a region of the game's world until he has reached a given point in the main storyline, or acquired a specific skill or object to open the "gate" (which is nearly the same in the case of such objects).

This type of gating is meant to stagger player progression over time, in order not overwhelm him with a too big world, and also rewarding him for staying invested in the game. Note that region gating is not incompatible with open world games. For instance, the GTA series and its cousins have been doing just that for ages.

Red Dead Redemption:
This broken bridge cuts you off from an entire section of the game world.

Once you've reached a milestone in the main story line, the bridge is automagically repaired.

Another reason for gating is when the locked region needs a specific skill or weapon to be played through. If you're doing things properly, that weapon/skill will be the key to unlock the gate. You can find that type of gating in the 3D Mario games, Darksiders, etc.

Darksiders: In Samael's hub, a tunnel is blocked by blue crystals.

Later in the game, you get a weapon capable of shattering those crystals.

Note that your gate does not need to be an actual obstruction. It can be as simple as a location out of reach without the required weapon/skill (which is a richer solution), or an NPC telling you that you can't leave/enter right now.

Encounter gating

Mostly used in brawlers, this type of gating consists in placing temporary obstructions that will prevent the player from leaving the fight before having slain his enemies. When combat is an important part of the game, this type of gating is set up to prevent the player from running away. Some games create an artificial arena (Ôkami), while some other simply obstruct the exits, (though often in a no less artificial way) (Devil May Cry,  God of War).

Devil May Cry 4: these cobwebs block the exits from the current area,
in which the payer can still freely move.

Ôkami: the red symbols form a wall around the player,
creating a very small arena in which the fight takes place.

This kind of gating is sometimes seen as bad design but is accepted as an established mechanic, so players usually don't have an issue with that. A clever combat system design may get rid of the need of encounter gating, by discouraging the player from running away from the fights (e.g. making him risk attacks from the enemies he's trying to get past).

You can get away without any encounter gating in games where combat is not a key part of the game experience, or where choosing the location of the fight is essential (such as Batman Arkham Asylum).

Encounter gating is also a good way to break the flow of speed-runners (assuming this is a good thing).

When using encounter gating, the player should be clearly notified that the area has been gated (using cutaway cameras, a distinct sound event such as a change of music, etc.). Obviously, when the gate is lifted, the player should be told as well.

"Synchronisation" gating

Used in cooperative games and/or games with allied AI, these are points you can't get past without your partner. The mechanic used is generally synchronised interaction points (a object that needs two people to be pushed, a point where you need to leg your buddy up, etc.).

The purpose is to enforce and reinforce cooperative play, by preventing a player from running alone to the end of the level.
It is slightly different in single player though, as (in a well designed game) the NPC should be ahead of you at the interaction point, in order to inform the player that there is something to do there.

Army of Two: Some areas require both players to get past, using a co-op move.