It's a 12 issue "event" that will happen in every single X-Book (so its just gonna last 4 weeks) that's gonna shape every big storyline for most of those books.
They using the Gala as an in-world event were everything related to mutants will be revealed to the world:
They will announce the new X-Men team (that hasn't existed since Hickman's run started, they got away with having an X-line without a legit X-Men team doing hero shyt for 2 full years).
They will reveal some intergalactic fukkery.
They will show possible conflicts with other nations.
Why a fashion gala? I guess because they wanted to try something new
who cares?