Fence Gates are the initial gates available to the player, allowing creation of enclosures in tandem with Fences for the protection of areas from both hostile and plant-hungry wildlife as well as raiding parties.
Fence Gates or higher-tier equivalents are required for fences to be passable by villagers and cattle in the case of a complete fencing enclosure without gaps.
Basic Strategy
Fence gates should be interspersed in long lines of fencing as needed for access, full fencing enclosures without gates will be inaccessible to Villagers or Cattle.
Gates (and their defensive equivalents) are frequently a prime target for entry into a walled-off settlement by raiding groups due to the slightly lower total durability versus walls and fences of the same tier.
This raider behavior can sometimes be exploited by stacking several gates in succession flanked by other defensive structures like Lookout Towers.