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.