Could someone gain height if she is 16 years old ? and if yes what should that person do in order to gain height ?

Possibly: At 16, you're on the edge of your growth curve. If your bony growth plates are still open, you could grow taller -- but most girls are finished with that by 16-17. This is determined mostly by genetics. You can take good care of yourself by eating well, getting healthy sleep, and exercising. That way your body will be able to flourish as much as your genes allow.