PEOPLE THOUGHT WE would move back downtown,” Christine Gachot says, sitting in the living room of the Sutton Place apartment she and her husband, John, share with their two sons, Boris and Jack, and Slim, their springer spaniel. After all, for 18 years, they had lived in a loft on Bond Street, until they outgrew it during the pandemic and did something that probably only two designers (they are co-principals of Gachot Studios) would think to do: “We took this little moment to live in the Paul Rudolph museum,” Christine says about having rented the architect’s disco-modernist triplex penthouse cantilevered atop an 1867 brownstone at 23 Beekman Place. It’s not actually a museum—if you want that, go to the Metropolitan Museum exhibition on the architect through March 16—but it’s a…