"Gates automatically allow friendly units to pass through the walls. Gates can be placed directly on already built walls (a portion of the wall's resources are recovered in that case)."
A Wood Gate is a defensive building available in the early game without research. Friendly units and troops can move through the gate, but infected must break it to pass.
Gates are useful for partitioning your settlement into smaller areas, while still allowing you to transfer units between them. If a small portion of your settlement is overrun, interior partitions allow you to contain the infection and defend against it. Likewise, an exterior gate allows units to sally forth to attack infected, but easily retreat to a protected position if they draw more attention than they can handle.
Because of their large size and therefore attacking surface, wooden gate have much less resistance to attacks than to wooden walls, despite their higher HP. Therefore they shouldn't be built directly in the line of the attack of infected, but rather sheltered behind a screen of other defenses. They are also more expensive than walls, and so they should only be used when they provide real value. Early on when infected hordes are small and the direction attacks will come from somewhat predictable, you can usually rely on the mindless infected to not be able to negotiate a complex path through your walls, allowing you to avoid the expense of gates until they are truly necessary.
Can be upgraded to a Stone Gate.