As a private park, Gramercy Park is held in common by the owners of the 39 surrounding structures, as it has been since December 31, 1831. Two keys are allocated to each of the original lots surrounding the park, and the owners may buy keys for a fee, which was originally $10 per key, but as of 2008 was $350, with a $1,000 fee for lost keys, which rises to $2,000 for a second instance. The Medeco locks are changed annually, and any property that does not pay the annual assessment of $7,500 per lot has its key privileges revoked; additionally, the keys are very hard to duplicate. As of 2012, there were 383 keys in circulation, each individually numbered and coded.
Members of the Players Club and the National Arts Club as well as guests of the Gramercy Park Hotel, which has 12 keys, have access, as does Calvary Church and the Brotherhood Synagogue; hotel guests are escorted to the park and picked up later by hotel staff. In addition, the owners of the luxury condominium apartments at 57 Irving Place, which was completed in 2012, will have key access to the park, despite being located several blocks from the park, by becoming members of the Players Club.
At one time, the park was open to the public on Gramercy Day – which changed yearly, but was often the first Saturday in May. In 2007, the trustees announced that the park would no longer be open for Gramercy Day because it "had turned into a street fair". The park, however, continues to be open to the public on Christmas Eve. Visitors to the park may not at any time drink alcohol, smoke, ride a bicycle, walk a dog, play ball or Frisbee, or feed the birds and squirrels.
In 2001, Aldon James of the National Arts Club that adjoins the park brought about 40 children, mostly minorities, into the park from the nearby Washington Irving High School on Irving Place. The trustee at the time, Sharen Benenson, called police alleging that the children were trespassing. The police refused to take action. Later, a suit was filed against the park's administration in Federal Court. The suit was settled out of court in 2003. Most of the children settled for $36,000 each, while one received $50,000.
In December 2014, it was revealed in The New York Times that 360-degree panoramic pictures of the interior of the park – taken using Photo Sphere, a Google app within Google Street View, by Shawn Christopher from the Pittsburgh area – had been posted to Google Maps. Christopher got access to the park by renting a room through the Airbnb service and using the key to the park which came with the room. The Gramercy Park Block Association – which did not know about the photographs until informed by a Times reporter – did not give Christopher permission to shoot in the park, and he was unaware that photography was generally forbidden.