The turret and portal are both on a viewscreen. Like you Bingo, I always thought the SD commander was looking at a viewscreen showing the open space outside the bottom hanger bay, with the gun being located inside the bay (so it is essentially a camera shot looking straight down, possibly behind the Blockade Runner, which is why we can't see it).

From what I've read, this was the original SD bridge (ala very similar to the Death Star bridge layout, with a large viewscreen in the center, maybe a console or two, with 2 circular consoles on the left and right sides).