The gate was the location of several fights during the War of Independence. If you look around the gate on its external side, you can see the stones of the wall punctuated by bullet holes and mortar shells, which hit the gate during that time. The Palmach fighters even tried to blow it up at one point in order to make it to the Old City, but this attempt failed.
Like the well-known Jaffa Gate, Zion Gate is also a Tafnit Gate, meaning entrance into the gate is only at a 90-degree angle. These types of gates were built to make it harder for enemies attacks to come straight into the city. The idea behind these gates is that anyone coming through has to make a turn while entering the city, slowing down any enemy attack exposing them to the city's protectors.
The name, Zion Gate, comes from the name Mount Zion, which is where the gate leads. This gate already existed on Crusader maps back in the 12th century, only named "Mount Zion Gate." In the British Museum there's a map from the 13th century, only named "Porta Syon."