“Gathering” is the title of the Guggenheim’s triumphant Alex Katz retrospective (on view through Feb. 20), but it might easily have been “Conversations with Friends”—and not just because a stylish detail of a Katz double portrait graces the cover of Sally Rooney’s novel by that name. (That daffodil-yellow canvas, from 2009, is among the more than a hundred and fifty works here.) Two subjects have absorbed the ninety-five-year-old native New Yorker across his eight-decade career: the people he loves—above all, his wife, Ada, and such close pals as Frank O’Hara, John Ashbery, and Meredith Monk—and the landscapes of Maine, where he has summered for some seventy years. One bright outlier in the exhibition, “Round Hill,” from 1977, finds his friends in the endless now of a Caribbean getaway. For years, Katz was something of an outlier himself. A figurative painter who favors the flatness of Ab Ex, his style is singular, neither Pop nor Photo-Realist. His sharp eye for fashion (a chic red lip, a patterned scarf, a snazzy pair of sandals) can be deceptive. Such details are to Katz what apples were to Cézanne (whom Katz has called “the first artist I understood”): an invitation to eye the interplay of color and light, load a brush with oil, and master the depths of a painting’s surface.