“Frozen River,” the Sundance-launched dramatic thriller set on the U.S.-Canada border, won two of the six prizes at Tuesday’s Gotham Independent Film Awards.The IFP’s 18th annual Gothams, hosted by “The Daily Show” correspondent Aasif Mandvi at Cipriani Wall Street, recognized the pic as best feature and star Melissa Leo as breakthrough actor. Acquired by Sony Classics after it screened in Sundance, “Frozen River” has become that rare thing in the 2008 indie/specialty world: a grassroots hit. Like “The Visitor,” which was also nommed for best feature at the Gothams, “Frozen River” benefited from coming out before the fourth-quarter prestige crush with a lot of fest buzz to get it started commercially. Released Aug. 1, the pic has cumed $2.3 million in the U.S. “Synecdoche, New York,” which lost out to “Frozen River” in the best feature category, won for ensemble performance. “Ballast” helmer Lance Hammer won the breakthrough director prize, and Zeitgeist Films’ “Trouble the Water” won for documentary. The “Best Film Not Playing at a Theater Near You” honor and a $15,000 cash prize went to fest fave “Sita Sings the Blues.” Tribute awards were given to Penelope Cruz, filmmakers Gus Van Sant and Melvin Van Peebles, and HBO doc maven Sheila Nevins. The Gothams have no technical connection with the Spirit Awards given out by estranged sibling org Film Independent. The Spirit noms, honoring many of the same films, were announced earlier Tuesday. A roster of 18 critics, journos and programmers picked the 22 films nommed in the six categories. The “Not Playing at a Theater Near You” trophy was decided by the editors of IFP’s Filmmaker magazine and a curator from the Museum of Modern Art.