Well that is a good question regarding whether it is manned, because the answer is yes and no.
Typically, there was an operator seated in a chair aft and central of the gun breeches. However, the Skyguard was capable of operating the guns autonomously; the gunner's controls could be overtaken and if memory serves, he needed be in the seat at all. This feature would be used when the Skyguard operator got a vampire warning light and would basically hit the panic button which would slew both guns on to the inbound missile and fire a combat burst of AHEAD ammo. Caveat - it's been around 15 years since I took my Twin 35mm Troop Commanders Course, so I might have some details wrong.
As for being completely enclosed, there was an umbrella shaped cam net that could be deployed around the gun. The gunner's station I don't recall having any enclosure, other than sides, back and bars resembling a roll cage. The gunner would be belted in, in case the FCU went autonomous, it could traverse the guns at something like 360* in just over 2 seconds. May not sound like a lot, but trust me, when you are sitting in the chair, its moving!