Puberty generally follows a genetic pattern in healthy well fed children. This includes gymnasts. Most girls start periods between 9 and 16, with 11.5-12 being average overall. Most girls follow either their mother or their father's side of the family's pattern - unless seriously undernourished. Gymnastics as a sport tends to have more late "bloomers" but in general if you ask the parents of the gymnasts, they tended to be "late bloomers" as well, even if not athletic themselves.
If a child athlete is growing slower/developing later than would be genetically expected, then its worth a Doctor's visit. It is true that the female athlete triad can occur in gym, but in my experience its uncommon these days (perhaps short of the super high hour programs - the average girl doing 20 hours a week of gym seems to develop along familial patterns). The female athlete triad can occur in gym, but I personally see it more in runners and occ. soccer....and more in kids 15-16+.
It is also unfortunately true that girls who go through normal but earlier puberty often quit gymnastics. Traipsing around in a leotard with your friends who are genetically blessed with later puberty while you struggle with hormones, fast growth and how it changes your skills, dealing with looking like a "normal 6th grader" versus your pre-pubertal friends, and periods, etc sends many of them off the competition floor. Certainly in our region by L10 most kids are past puberty, but in the mid level optionals there is a strong selection for the later bloomers....most of whom could eat my family out of house and home and still grow slowly!
In general, gymnasts have STRONGER bones than other kids their ages - but the skills and difficulty done while also growing can lead to fractures at this age. The kind of fracture (a "normal" type for the activity involved versus unusual) can also guide you as to whether to be concerned or just frustrated!