The architect Charles Gwathmey has died at 71 after a prolific and bewilderingly erratic career that, despite his absence, isn’t quite over yet. Some of his major designs are still under construction, including the muscle-bound U.S. Mission to the United Nations and a doggedly hyperluxurious Fifth Avenue residential tower. In the sixties, Gwathmey was a member of the New York Five, a gang of swaggering modernists that included John Hejduk, Richard Meier, Michael Graves, and Peter Eisenman. Gwathmey was the most urbane and adaptable of the bunch. Along with his partner Robert Siegel, with whom he founded the firm Gwathmey Siegel & Associates, he supplied castles on the beach to the East Hampton nobility, corporate high-rises, apartment renovations, as well as a stream of civic projects. Rather than brand his designs with signature mannerisms, he combined smooth elegance of execution with the willingness to be self-effacing. In revisiting Frank Lloyd Wright’s Guggenheim and Paul Rudolph’s Art and Architecture Building at Yale, he politely declined to compete with the masters. Unfortunately, when some of the juiciest projects came his way, he occasionally allowed himself an infuriating lapse of taste.