One of the main features of Patch 10.2.5 is the questline detailing Gilneas being taken back by the Alliance. We did the questline and document everything that happens in it!
The questline starts for Alliance characters with a Greyguard Elite near the portals in Valdrakken — Talk with him to start the quest Summons to Lord Greymane, asking you to meet up with Genn Greymane in Stormwind Harbor.
In the quest To Gilneas, Greymane announces it is time to take back Gilneas, but the Scarlet Crusade has moved to the lost city to use it as a stronghold. As such, we're moving in to kicking them out. While we fly to Gilneas with Genn and Mia Greymane, Greymane talks about the Scarlet Crusade — Specifically, about how he has little experience with them, as they rose to power after Gilneas close themselves off to the rest of humankind.
Horde characters start the questline with Clandestine Movements, and are requested to meet with Calia Menethil and Lilian Voss at Pyrewood Village in Silverpine Forest.
In the quest To Gilneas, we learn that Lilian has been in contact with Tess Greymane due to their connection via the Uncrowned, and using that connection to return Gilneas to the Alliance — but the Scarlet Crusade has taken residence in there. As they're a common enemy between the Horde and the Alliance, Lilian is more than happy to help with killing them off.