i don't know: movie Gundabad looked too thin and empty. like it was just a bunch of angular, wedge-shaped spires overlaid with scrap metal (like the EVERYTHING-MUST-BE-IRON Horde from World of Warcraft: Warlord of Draenor). i know it's just the eastern gate, but it still looks wimpy compared to the Morannon, which was made by Gondorians at the height of their power, with the sharp, pointy "Iron Horde" additions made later by Sauron.
if we're going for any kind of fortress (personally, since its up in the north, i always saw Gundabad as just being a cold, ultimate North wasteland with orc camps, maybe some ancient Dwarven ruins, but no large fortresses or anything resembling civilization), then maybe something Dwarvish and ancient, since it was a meeting place for the Seven Houses of the Dwarves, but with the iron on top of that.