File this under “only in New York”: Activists are thumbing their nose at a $113 million gift to the city and trying to kill plans for a gorgeous new park on the Hudson.

And this time the donor isn’t even those darn Koch Brothers — it’s entertainment mogul Barry Diller and his fashion-goddess wife, Diane von Furstenberg.

As The Post reported Wednesday, the City Club of New York is demanding mountains of documents as part of its suit to stop Diller’s “Hollywood on the Hudson” — a 2.7 acre recreational refuge on a platform over the Hudson River called Pier 55 and funded almost entirely with donated money.

The demands are designed to slow progress — even if the suit fails.

But why oppose this gem of a gift? The City Club claims the project needs more environmental review and public input. Typical. New York naysayers always want more “review” — death by eternal delay.

In fact, the plan’s gone through both environmental analysis and public review. The local community board gave its blessing, as did Mayor de Blasio and Gov. Cuomo.

After all, the park will be an obvious boon, with new walkways over the water, a wider esplanade, grass fields and groves.

The centerpiece: a 700-seat amphitheater to host arts events and top-flight entertainers under programming led by renowned producer Scott Rudin.

And most activities will be free to the public, or low-cost. So why oppose it?

But then, this handful of city-planning insiders doesn’t need a reason to fight the plan, as long as it’s not their plan. Indeed, the City Club helped kill then-Mayor Mike Bloomberg’s efforts to save a pillar of city commerce by rezoning Midtown East.

Diller and his group deserve the city’s gratitude here; the court should toss the case ASAP.