In a sign that Gristides billionaire John Catsimatidis is muscling up for a New York mayoral run, a friend, lawyer, lobbyist and communications specialist Brad Gerstman, has bought a home on the Upper East Side. We’re told Gerstman, a founding partner of Gotham Government Relations, has just closed on a pad on 72nd Street to focus on the grocery-store magnate’s mayoral race. A source told us, “John is seriously doing the rounds to sound out people for his run, not just the politicos but regular New Yorkers.”