This event can still make you some decent gold, but it is not nearly as profitable as Palawadan now, thanks to a nerf ArenaNet put in place ending multi-map looting Great Hall when your character has not completed the event. However, I will still cover the basics of how to participate in this event, and how you can still farm it more efficiently between Palawadan raids.
Great Hall is in some ways just a shorter version of the Palawadan raid, in that there are a few event phases that upon completion spawn Supply Stashes, and champion enemies to kill for their Phylacteries. However, the biggest difference is the nerf ArenaNet put in place that requires your specific character to participate in an event phase in order to be able to loot the respective supply stashes. What this means is you cannot map-hop to another instance that has the event in progress and loot the stashes there, and you also cannot log in on an alternate character and loot the stashes you unlocked with your first character. This effectively forces a maximum limit of 12 stashes per Great Hall event per account, which pales in comparison to the dozens you can get from Palawadan. Nevertheless, an efficient zerg could still participate in up to 4 or 5 Great Hall events in between each Palawadan, which could net you 48 to 60 stashes. The choice is yours whether you want to participate in Great Hall. For those who do, read on below for the event details.
GATES: Picking up where we left off in our Palawadan example, the Palawadan raid started at 2:45 PM and ended at 3:15 PM, with an additional 5 minute escape period that ended at 3:20 PM. No matter which instance of the map you're on, the Great Hall event will always begin immediately once the Palawadan escape phase ends (in this case, it starts at 3:20 PM). This effectively syncs all Great Hall events across all instances of Istan... for now. We'll get to how you can take advantage of them de-syncing later.
The total duration of each Great Hall event is 15 minutes, and will conclude with an additional 1 minute escape phase. For now though, at the beginning of the event, the zerg needs to break down the main gates while fighting off numerous mobs, veterans, and a champ enemy or two. Just DPS the gate and tag any champs you see until the gate breaks open and you can collect the first 2 stashes in the tents inside and to the right.
COURTYARD: This next phase is a sweep-and-clear operation where the zerg floods into the Great Hall and obliterates all the mobs. There aren't really any champs in this phase, save for any left over from the gate phase that were not yet killed. Remember to check the back side of the Great Hall for slow mobs hiding out that need to be killed before the next phase starts, otherwise you'll be waiting longer and won't be opening stashes as quickly. Once the mobs are all killed, 2 stashes will spawn in the small tents right next to the central courtyard staircase.
CAPTAIN: Once the courtyard is clear, a champ captain will spawn with some additional veteran mobs, as well as one champion enemy on either side of the central platform. The captain has the swirly-attack, so CC his break bar at 75% and 25% health to avoid his damage spike. Get some hits on the additional champion enemies if you can to get more Phylacteries. Once the captain is dead, 2 more stashes spawn on the central platform.
FOUNTAINS: The next phase has the zerg heading inside to some relatively confined spaces to destroy the mesmerizing fountains of Joko while subduing the brainwashed Sunspear recruits. Nothing fancy here, just DPS everything in sight, and if you want to PRAISE JOKO go ahead and interact with the fountains a bit. When all fountains are destroyed, 2 more stashes spawn, one on each interior wing where the outer fountains were. The large door to the central chamber opens, and you're on to fight Amala.
AMALA: This fight is a bit of a pain. It's not that bad with a huge zerg that can constantly resurrect people, but Amala will do her best to confuse you by spawning dozens of clones of herself, dropping portals all over the floor, constantly jumping from one side of the room to the other (frustrating melee fighters like my warrior to no end), and when you're least expecting it, dropping a giant freaking meteor on you.
When you first enter the room, it's helpful to target the real Amala (not a clone of hers) so that the red bull's-eye will make it easier to follow her around the room. Do your best to put damage on her while her break bar is unavailable. There will be specific moments in the fight when she stays put for a little bit and her break bar becomes active, and the commander will usually call out "CC" at that point to call for everyone to focus on dropping that bar. Make sure you're ready with CC when that time comes, and if not just continue to dump DPS into her.
As her health is chipped away, she'll change up the battlefield by doing things like bringing in a champion elemental, cordoning off the central part of the room, or dropping meteors on you. For the elemental specifically it's recommended to ignore it because it does NOT drop a Phylactery and will de-spawn once Amala hits a certain health level and changes the battlefield again. However, there will occasionally be champ enemies that spawn near the corners of the room which DO drop Phylacteries, and some of the zerg may break off to focus those down quickly before turning back to Amala. Watch for "champ" callouts and follow the instructions of your commander to defeat her and spawn 4 more stashes in the central chamber.
NEW MAP / 15 MINUTE BREAK
This is where things get slightly more complicated for the Great Hall event. In order to help describe it, I've put a reference chart below denoting the times each event type starts and stops across multiple maps as a simplified example of how this works.
An example schedule of events in Istan occurring on multiple map instances.
With multiple instances of Istan running at the same time, it's possible to get a zerg to hop maps to where a new event may just be underway in order to increase the loot per hour of the participants. This is really the only way to make the Great Hall event profitable while you're waiting for Palawadan to start again.
To understand how this works, let's go back to the end of Palawadan. At 3:20 PM in our example, the escape phase of Palawadan ended, and the Great Hall event just started. That happens at the same time on every map instance of Istan. A zerg continuing to farm after Palawadan will participate in the Great Hall event on the same map as the Palawadan they just completed. Great Hall runs for 15 minutes with a 1 minute escape phase, so it will end fully at 3:36 PM. However, during this first Great Hall after Palawadan, if a zerg completed Great Hall fast enough, there may be time for it to hop to a new map to catch the tail end of another Great Hall event. Everyone who jumps maps will no longer see supply stashes already waiting to be looted thanks to the nerf, but instead you can participate in the last few phases of the event to improve your loot at bit.
Once 3:36 PM hits, every Great Hall event across every map instance of Istan is finished. After each completed Great Hall, there's a 15-minute "cool down" before the next "Sunspear Uprising" pre-event to Great Hall begins. So between 3:36 PM and 3:51 PM, there's no Great Hall event anywhere, and the zerg and commander will likely take a short break before the next slate of events.
GREAT HALL MAP TRAIN
Once the "Sunspear Uprising" event pops around 3:51 PM though, things get interesting. The commander will lead the zerg to start making progress on the event bar to push it down quickly. Once the bar is empty, the Great Hall event starts, so everyone should be participating in making that happen faster. If we make general assumptions here for example's sake, let's say the zerg completes the pre-event on their map (Map 1 in the graphic above) in 5 minutes, which would start the Great Hall event at 3:56 PM. On the other maps there may not be a zerg making fast progress on the pre-event, so it takes a much longer time for the pre-event to be completed. This works in favor of the zerg because once it finishes the Great Hall event (again assuming 5 minutes to finish, so completing at 4:01 PM now), it can hop to a new map with the pre-event still active, complete the pre-event, then do Great Hall all over again for more supply stashes.
This is the pattern you see repeated through maps 2, 3, and 4 above. Although this is a simplistic illustration of the concept, and in reality map-hopping is complicated by other factors, this should describe the goal fairly well, and how map hopping actually makes it work. Using this method, I'd say at best you can finish a total of 5 Great Hall events between Palawadan raids, but usually only 4, hence the estimate of 48 - 60 stashes total between raids.
One quick note though is that at 15 minutes before the next Palawadan (4:30 PM in our example), the Great Hall event is prohibited from starting on any map instance where it is not already started. What this means is if you're working on the pre-event at 4:29 PM and the clock strikes 4:30 before you complete it, then even if you complete the pre-event a minute later, Great Hall will not start. This is a buffer built into the map's event cycle so that Great Hall doesn't manage to overlap with Palawadan. You'll actually see a notification on your screen that says "Sunset approaches. The raid on Palawadan will begin soon." This is a heads-up that Palawadan begins in 15 minutes, and also signals that Great Hall can no longer be started before the raid.
The notification that Palawadan will start soon. Note the time on the mini-map is at 4:30 PM.
Before wrapping up the Great Hall section, let's cover a couple more quick things.
SUNSPEAR UPRISING PRE-EVENT: This pre-event to Great Hall has a meter that diminishes with every enemy killed in Istan, as well as with every Corpse Caravan destroyed before reaching the Great Hall. When the pre-event starts, the commander will usually lead folks inside the Great Hall, using mounts to jump or fly over the wall, and start farming the mobs inside to push the bar down. Some zerg members will stay outside the Great Hall, keeping an eye out for Corpse Caravans when they spawn and heading out to intercept them. Make sure you're participating in either the mob-killing or caravan interception to help finish the pre-event as quickly as possible.
HELPING FIND A NEW MAP: If your zerg finishes a Great Hall event and the commander asks for help finding a new map, the easiest way to do that is to log out to the character selection screen, then log back in on your same character. By doing that, there's a chance that the game places you in a different map instance than the one your zerg was previously on. If you find yourself on a new map instance with an active pre-event or Great Hall event, let the commander know in squad chat so they can organize the group to move to the new map. If you don't find a new map on the first try, you can keep trying indefinitely. Having commanded several Great Hall trains myself, I can say with authority that the help finding new maps is greatly appreciated!
At that point, you'll quickly lose health automatically, be killed, and teleported out of Palawadan. But it's OK, because you got lots of loot, and even better, the Great Hall event starts immediately!