'their gathering' is correct and usable in written English.
You can use it when referring to a gathering of people that belongs to someone or something. Example: The cheerful guests attended their gathering in the garden.
The fire station is their gathering point.
The Apache used their gathering time to express gratitude.
All they knew was that "a powerful ally" would be attending their gathering, Ms. Soltani said.
"The AIDS patients are in their gathering place, where they get their drugs," Mr. Wilson said.
Their gathering was a wake, of sorts, but one shot through with hope.
Maugham's focus is on a country solicitor's family and their gathering misfortunes.
Nevertheless, the sheer multiplicity of sights made their gathering lively as well as enigmatic.
Then, at the urging of The New York Times Magazine, their gathering became more productive.
They started at 7 a.m., which gave their gathering its name: The 7 O'Clock Club.
But Ireland's reaffirmation of their gathering power was a limited success.
In their gathering desperation, this community of lawyers and their comparably grasping claimants feels absolutely time-honored.
