Gates are a barrier meant to block enemy troops from passing through while allowing friendly troops from entering, and is usually coupled with walls as an effective defense.
Gates have two variants; one that faces horizontally (east/west) and one that faces vertically (north/south).
The Allies, Soviets and Epsilon use different model of gates, although their stats are similar to each other, like walls. The Foehn do not use gates but their Stun Grids and Turmoil Grids fulfill the role of a gate to some degree.
Gates are simple defenses which allow a commander's units or his ally's forces to move past a base's defenses while keeping enemies out. Best used at choke points or for vulnerable sections of a base.
Automatic gates are used to keep enemies out while still allowing friendly units to pass by defenses. Great for prying/keeping spies from getting into your base too easily, but don't rely on them too much.
Construction of Gates are often situational in normal battles, especially in open battlefields where enemies can simply move past these barriers. Therefore players should only construct them in a chokepoint just outside their base, if there are any. Gates are only a barrier to unarmed units (like the Engineer) and small strike forces that are meager against the Gate's heavy armor. While Gates are hardy and can be repaired, it is only a nuisance against more formidable armies, as they will either breach the Gate itself or adjacent walls (since they cannot be repaired manually) with ease.
Behind the scenes
The Epsilon Gate's original artwork is available for public download to be used by other modders. The link can be found here.