The name is also given as Dunn, but I think that's easily explained by someone mistaking the "G" in Gunn for a "D." I've read Moreno's account and also have copies of a number of his original letters that I found in the National Archives that were not included in the Official Records set. I've been able to locate all of the battery sites up and down the River, including the one at the Narrows which is now well away from the river thanks to a change of the channel.
The source of overblown claims of what was captured at the arsenal was actually a Savannah newspaper of the time. I think they were just making good copy by inflating the amount of gunpowder, etc., seized there.
Have you ever had a chance to visit the arsenal? There is quite a bit of it left, although much of it is in bits and pieces.
Dale