A Gate is a portal to another world, where Monsters can often come through.
Gates are specific to locations that have unique pictures by their name. This includes cities like Rome or Tokyo, or places where an Expedition Encounter can take place, like The Pyramids or Tunguska. Mountains of Madness and Under the Pyramids have side boards that can also have Gates opening.
All Gates have one of three symbols on them that correspond to the Omen. When the omen rotates to that symbol, Doom advances by 1 for each Gate that bears the symbol. Gates do NOT advance Doom when spawned, even if the Omen corresponds with that Gate.
A Gate may be encountered with an Other World Encounter during the Encounter phase if the player wishes. Typically, one skill check is needed initially, and the player gets two different encounters depending on whether or not they pass or fail this check. Some encounters, however, allow the player to choose which of the two effects to resolve.
Many Other World Encounters allow the player to close the Gate. If this happens, the gate is removed from the board and returned to the bottom of the stack of Gate markers. Other effects like Conditions may allow the player to seal a Gate without the Other World Encounter. If playing with them, a Mystic Ruins Encounter will often let the player seal a Gate, even if they aren't on the space of that gate.
Gates are most commonly opened when spawned by a Mythos card when the symbol appears on the Mythos card, always occurring on the cards with yellow backgrounds. They may be opened through certain Rumor Mythos cards, or through some condition effects.