host of “The Biggest Loser” and celebrity trainer, is listing his longtime New York City pad for $2.3 million, The Post can report.Harper, 56, initially purchased the one-bedroom, two-bathroom abode for $1.895 million in 2015 — when he took over as host of the fan-favorite show, succeeding Alison Sweeney. But it seems he is now ready to let go of the modern industrial designed apartment, two years after he got engaged to longtime boyfriend Anton Gutierrez.Pegged as a “boutique condominium,” the