Our gym starts adding ballet to the practice schedule once the girls are in the pre-team program. So every girl on team and pre-team gets 1 hour/week of ballet. Our previous gym that has trained girls up through elite does not have any formal dance instruction and when I did ask my gymmie's coach there about having her take a ballet class, the reply was, "it will only mess her up."
Not all gyms will require it. Mainly because they dont have the appropriate teachers to teach it or because they dont have enough hours to include it. But it is an excellent supplement to their gymnastics training. Ballet is a huge part of their floor and beam routines. Thise with dance training have a distinct advantage.
My gym doesn't require the gymnasts to do it. Another gym in my area does. They go to a dance studio as a team once a week, and that is the first part of their practice (they go to the gym after). They seem to have better floor routines then the girls at my gym.
My DD is taught ballet like moves (mostly barre work) and arm/hand movements by her coach. This is necessary as there is an artistry element in floor and beam scores here (we use FIG scoring system for all levels). Ballet is an art form and this is artistic gymnastics so they have a lot of cross over. A gymnast with good artistry is a joy to watch!