The reference is to the U.S. detention station at Guantanamo Bay in Cuba, where people arrested in the war on terror have been taken. This is a politically charged term; others insisted that Pier 57 was humane and was more like "Club Med on the Hudson."
Wikipedia: Pier 57
Pier 57 is a long pier built on floating concrete caissons in the Hudson River in Manhattan, New York City. Built in 1952, it is located near the end of 15th Street on the West Side Highway, just south of the Chelsea Piers sports complex.
About one year after the NYCTA vacated the pier, Pier 57 was temporarily utilized as a detention center during the 2004 Republican National Convention, when approximately 1,200 anti-RNC protesters were arrested and sent to a makeshift detention/processing center at Pier 57. Over 1,800 were arrested during the entire event. Medical activists reportedly treated many people held at Pier 57 for chemical burns, rashes, and infections that resulted from direct, prolonged exposure to the motor oil, asbestos, and other contaminants from its days as a bus garage.
31 August 2004, New York (NY) Times, section P, pg. 12:
THE REPUBLICANS THE CONVENTION IN NEW YORK - ARRESTS
Demonstrators Held at Pier 57 Complain of Conditions and Long Waits
By PATRICK HEALY and SUSAN SAULNY
Pier 57 was once just another dingy hulk on the Hudson River, a three-story parking garage for New York City Transit buses. But this week, the pier near West 14th Street has been outfitted with benches, portable toilets and chain-link fences to hold its new freight: protesters.
People who are arrested protesting the Republican National Convention are handcuffed and taken to the pier to be held and processed before they are sent to Manhattan Criminal Court. As the city's temporary detention center for demonstrators, the pistachio-colored building jutting 700 feet into the river has become one of the more contentious emblems of security during the convention.
As of last night, 560 people had been arrested since Thursday, the vast majority for misdemeanors, and many who cycled through the system complained that they had been held for as long as 30 hours in miserable conditions before being arraigned or receiving a desk appearance ticket. Several said they had contracted rashes from sleeping on the pier's floor, had gone hours without food and were given a Dixie cup to use to drink water.
Describing their treatment, several protesters who had been arrested referred to Pier 57 as Guantanamo on the Hudson.
1 September 2004, Daily News (New York, NY), "WHINERS, WUSSES & WIMPS" (editorial), pg. 36:
In this case, well, these abusive jailhouse conditions are just shocking. No vegan sandwiches, just baloney. Nothing but Dixie cups to drink from. Small wonder they're all calling the holding cell "Guantanamo-on- the-Hudson." Where are the international human-rights monitors, Amnesty International and the like, when you need them?
Boo hoo hoo.
This "Guantanamo" is a Transit Authority bus garage on Pier 57 where protesters are detained and their property collected before they head off to Central Booking. With an oil stain or two on the floor, it admittedly is not the tidiest place in the world. But the Black Hole of Calcutta it isn't. Still, this morning there was scheduled to be a seriously indignant demonstration outside the building to object to all that unendurable barbarism within.
2 September 2004, The Times (London), pg. 17:
Police hold 1,000 at "Guantanamo on the Hudson"
3 September 2004, The Independent (London), pg. 33:
Republican Convention: "Guantanamo on Hudson" awaits demonstrators
22 November 2004, Associated Press Newswires:
Lawyer: "Guantanamo on the Hudson" housed RNC protest arrestees
By LARRY NEUMEISTER
NEW YORK (AP) - Saying New York created its "own little Guantanamo on the Hudson" during the Republican National Convention, a lawyer Monday filed a lawsuit seeking relief for nearly 2,000 arrested protesters and bystanders.
"All that was missing were the orange jumpsuits," lawyer Jonathan C. Moore said
27 November 2004, New York Post, "'LITTLE GITMO' GUANO" editorial, pg. 16:
Last Monday, the Center for Constitutional Rights filed a lawsuit in U.S. District Court seeking class-action status for the nearly 2,000 people picked up by the NYPD at protests during this summer's Republican National Convention.
The fact is that the cops did a fantastic job during the convention, under very difficult circumstances.
But New York was never "little Guantanamo on the Hudson" - as the suit so hyperbolically asserts.
30 November 2004, New York (NY) Sun, "Mental Anguish" editorial:
During the Republican National Convention, Mayor Bloomberg and the Police Department created a "little Guantanamo on the Hudson," attorney Jonathan Moore alleged last week, referring to the military detention camp in Cuba.
We have some reason to be suspicious of these charges. The New York Sun's own Josh Gerstein was detained at Pier 57 during the Republican convention. He found the facility to be clean and comfortable, with benches and portable toilets provided to arrestees and carpets covering the floors. A spring-water dispenser was also accessible.
"They're calling this 'Guantanamo on the Hudson'? It's more like 'Club Med on the Hudson,'" said one arrested activist, Andrew Coamey. Mr. Coamey, who works for an AIDS housing nonprofit, Housing Works, was among 19 people arrested during a demonstration at Grand Central Station. He said conditions in a crowded precinct lock-up would almost certainly be worse than those at Pier 57. Besides, Mr. Coamey said, those who intend to be arrested should expect to endure a bit of discomfort for their cause.