Gates aren't upgraded directly, but are improved when the corresponding Wall is upgraded. A Medium Wall Gate has 2750 hit points, while a Hvy Wall or Shield Wall Gate will have 4000 hit points (with the latter also being shielded). Unlike Turrets, Walls and Gates do not change appearance when upgraded.
"Walls keep your enemies out — but they keep you in. Therefore heavily fortified Gates can be built into your Walls starting at Tech Level 2, and these will allow your own troops to move freely in and out of your base, but will remain shut when hostile forces approach. They may also be locked or unlocked, to better suit particular situations, such as to prevent workers from wandering outside your base in search of resources. To lock or unlock a Gate, select it, and click on the Lock/Uniock button on the interface."