Cool project!
In the books, Gamling was the (old) Captain in command at the Deeping Wall. He was not mentioned later. In the movies, PJ gave him the role of Guthlaf, Theoden's personal banner bearer. Guthlaf, however, was killed at the Pelennor. Gamling's profile could very well represent the carrier of the King's banner though!
Cirion is made up by GW, so it's up to you to decide if you want to include him or not. Beregond should also be with Faramir, because he was banned from Minas Tirith by Elessar (for killing in Rath Dunen).
Damrod was a Ranger of Ithilien, and his profile represents this. Therefore, I think he's less appropriate in a Minas Tirith army (unless allied).
I think Imrahil would have stayed in Dol Amroth -as he was the Prince of Dol Amroth-, but he likely rode to war with Aragorn.
Forlong was killed at the Pelennor. Duinhir's sons (Duilin and Derufin) died at the Pelennor too, but he is not mentioned amongst the fallen, so he survived the battle. Angbor must also still have been around.
At the time of the WotR, Erkenbrand was Lord of Helm's Deep. So yes, it makes sense to give him Helm's Deep afterwards. I would keep him out of the Glittering Caves army list though, as I expect the Dwarves would be led by their own Heroes (otherwise, Captains of Rohan could be chosen with KG, to minimise the points spend on Heroes, for example).