The Tall Gate is a furniture item. They are essentially tall, sliding Doors, visually moving upwards when toggled with a right-click, such that an item placed on either side will not prevent it opening. They do not, however, actually occupy space above while open. Tall Gates have no other special properties to distinguish them from doors, and come in only one variety.
Although it may seem like a gate would be better at keeping hostile mobs out during a blood moon, Tall Gates are in fact easier to break open, as they cannot be proofed against forced entry in either direction by barricading with otherwise passable torches or furniture.
Because Tall Gates are only 5 blocks tall, most hostile NPCs can simply jump over them, even with the extra block above them that is required to place the gate.
Tall Gates are ideal for players who do not wish to dismount, but still use doors in their construction plans. This because a normal door is 3 blocks tall while a player riding a mount is 4 blocks tall (5 when on a Unicorn) and thus cannot pass through. Likely this is why the Tall Gate was introduced.
Despite the item sprite showing it from the front, like other doors, the Tall Gate can not actually open to the side when in use.