Most Helpful Girl
You can, but not straight away, especially if she's mid workout. Maybe start up a small conversation every now and then, speak to her abit more, then eventually when you've both got a grasp on each other, ask her for her number/offer yours
Most Helpful Guy
Sure, but maybe not abruptly or right away. Most people go to the gym for theirselves I think. Maybe if it's a girl you see regularly and you can strike up a little conversation here and there that doesn't go awkward.