So, the high impact of gymnastics is well documented to increase bone density and at least according to health researchers this is seen as a good thing. Coaches may view it differently. I just googled bone density and gymnastics and lots of stuff came up I saw several articles that actually recommend gymnastics as a pre puberty activity for girls as a way to increase bone density to assist later in life. Also, is there really any way to tell someone's bone density by looking at them? Maybe I have no idea what I am talking about but I seriously doubt that you can. You can see their frame and general body shape, but the use of the term bone density seems an odd thing to point out. I would think maybe they are referring to her frame or shape rather than her bone density and just trying to be more sensitive, but I guess you never know. What do I know, really.
The point is this.
Good or bad, many many coaches and programs look at body shape, frame size, strength to size/weight ratio when choosing gymnasts for their team and certain body types may have easier or earlier success. Almost all programs have some sort of criteria they use when selecting JO team kids, some more strict than others and what those things are will vary program to program. It is also the harsh reality of this sport that not every kid is cut out for the most competitive tracks of this sport for many different reasons. With that said, there are all types and body shapes at all levels of gymnastics, including elite gymnastics. Being smaller and light and super strong for your size, makes gymnastics easier, but that doesn't mean it isn't accessible to those outside that pseudo ideal. Some of the best gymnasts on my dd's team are not of the tiny and light variety.
Different programs approach selecting team members differently. If one doesn't work, then look for another as the experience at different gyms is most certainly different is all kinds of different ways.
But, please don't put the pursuit of elite gymnastics as your bar as to whether it is worthwhile to do gymnastics or not. There are so few elite gymnasts and so so many wonderful things about gymnastics and what it teaches these kids at every different level.
If she enjoys gymnastics and wants to pursue it, then find a program that embraces her and wants her and let her enjoy it. If high level gymnastics is in her future, it will take care of itself, assuming she is in a program that can support that. And if a few years down the road, she doesn't want to do it any more she will have all the lessons and strength and body awareness and coachability that will make her excellent at whatever she wishes to pursue after that.
And at six, it isn't about looking that far down the road. At the point when there is a large group of six year olds in a group, you just have no idea which of these kids will be there when they get to level 8 9 or 10. Of the 20 kids my dd started with, three of them are still with her. You may be able to see the ones that could get there easily under the perfect circumstances, but in my years of experience, the ones you think are a shoe in often aren't and many more just surprise the pants off you. Take this time to pursue lots of her interests and allow her to love whatever it is she loves doing.
Good luck!