As stated by others there was no main gathering spot. I assume you're asking because you want to be able to easily form a team. If that's true, then you'll want to get on the server's global channel. Use that to announce the arc, SF, trial, etc. you're looking for or are forming.
However, if you are looking for specific things like DFB, then the trainer in Mercy was the typical place for that. For AE, I'd say Cap's AE building and/or the Black Market. For SFs, Sharkhead near the ferry was pretty popular as there were 3 SF contacts there (Renault, Mantis, Kal). For ToTing, then GV was the place.
There weren't many costume contests, especially after the Mercy / tutorial remake because Longbow took over the fort.
For RP, I saw a lot of folks hanging around St Martial's Giza casino.
Other than that, you had the coop zones (primarily at higher levels) like Cim, RWZ and DA.