point of entry to a space enclosed by building, wall or fence
A gate is a place between two parts of a wall, or a fence which could be opened and closed to let people go through it.
In the Middle Ages, a city gate also allowed people to go into a (walled) city.
Nowadays, people also talk about gates at airports. When going on an aircraft, people need to get together in a certain area of the airport first. They then pass through some kind of doorway to get onto the plane. That is what is called gate.
Ishtar Gate is the oldest city gate in existence.
Gates to official residences often feature elaborate ironwork.