I saw those same pictures, and didn't see the movie Gate as it appears in the concept art, so not sure exactly where the idea came from. And regarding our day trolls, just how was the Gate opened when it was under Gondor's management?
Hmm, the Brownlands once were the abode of the Entwives, and so maybe the Gates were opened by Ents. Will have to ask Fangorn the next time that I see him - at least that would explain why he and his brothers were reluctant in TTT to go to war, as the last time they ended up being door wards afterwards, and poor ones at that.
Note that while rereading the books I'm finding more reasons why certain things in the movie stick out as odd.