Q. What is it supposed to be on the show?

A. A bookstore.

 
Q. Where is it supposed to be on the show?

A.  In New York City.

Q. When did we see it on the show?

A. In Episode 6 of Season One of Resurrection.

The scene starts with Dexter looking at photos of the Gemini Killer's victims, which are always posed in pairs. When he sees an image of a father and his teenage son, murdered together, it is so reminiscent of himself and his own son, that he is determined to end Gareth.

Wearing a dark suit (for one of the few times in the Dexter franchise), Dexter follows Gareth into a bookstore, and up the stairs to the second floor.  
 


There, Dexter watches as Gareth takes a book from a shelf and then places a bookmark into the book before closing and replacing it on the shelf.


Once Gareth has left the area, Dexter retrieves the book, and sees that it is about infamous serial killers. And the bookmark has been placed on the chapter about the Gemini Killer.  
 

Dexter correctly concludes that Gareth is an egomaniac, but when he flips the bookmark over, he sees that a time has been written on the back, and assumes it is meant to arrange a meeting with someone else. He wonders why Gareth is using a Cold War espionage tactic like a “dead drop”.

But Dexter hasn't been careful enough during this bookstore visit, because Gareth has spotted him, follows him, and when Dexter gets home, he finds Gareth waiting for him there.

A. This is McNally Jackson Books, located at 4 Fulton Street, in the Seaport historic/entertainment district.  It's between the New York's financial district and the Manhattan end of the Brooklyn Bridge.

It's located on the south side of Fulton Street, between South St and Front St. 
 
This is just a very short walk from the location where Harrison stops at an NYU booth and picks up a pamphlet about the university.

( You may have heard of the Fulton Fish Market. It used to be located across the street from the bookstore. But the building housing the bookstore and other shops is known as Schermerhorn Row, and is a historic structure dating back to 1812. ) 

The Seaport is a historic district located in Lower Manhattan on the East River, offering waterfront views, dining, entertainment, and the South Street Seaport Museum (with its collection of historic ships and nautical memorabilia). It was once a major New York port. Today, it's a popular destination for its restaurants, shops, concerts on Pier 17, and views of landmarks like the Brooklyn Bridge.

Here is a matching StreetView of the bookstore: 
 
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Here is an aerial photo of the location.  And here is a map link.