Architect Charles Gwathmey, who founded Gwathmey Siegel & Associates with Robert Siegel, died yesterday in Manhattan. Gwathmey was 71 and died of esophageal cancer, his stepson, Eric Steel, said. Projects completed by his firm included the International Center of Photography in Midtown, the Northgate apartment complex on Roosevelt Island, the Museum of the Moving Image in Astoria, Hoboken’s W Hotel and an addition to the Guggenheim Musuem. Gwathmey himself designed houses for a number of celebrities, such as Steven Spielberg and Jerry Seinfeld.