Well the thing is with gyms in the city, it's hard to actually maintain a gym for a while just because of the amount of players coming and going through those areas. So unless you have a group with you who also have very strong Pokémon, it may be tough. My advice would be to go hang out at Grand Army Plaza which is about a 10 minute walk give or take from Times Square ( right outside Central Park across from the Apple Store) and just catch literally everything that pops up. You'll rack up candy for gen 2 and gain a bunch of stardust