I'll skip through all of the qualifying statements about probing for the real reason your dd wants to quit, as I'm sure you'll do your best to understand, along with her, what's taken the fun out of the sport. With that said........
Being good at something, just about anything you can think of, requires hard work, goal setting, and self discipline. These are qualities your daughter has proven to possess just by the level she's reached at age 11. IMO she has developed many life skills that you can take pride in as her mother, as you were always there to guide her along the way. I can see where it would be so hard to let her toss that all aside when she shows so much potential, but.......There's a difference between being good, and that's where she's at, and being great, and that's where you'd like her to get to. No problem with that, it's just part of being a parent.
Just as being good requires certain things, reaching the level of "great" requires so much more. It isn't enough to work hard, set goals, and be disciplined in your work. The additional assets of a great gymnast must include talent, such as your daughter's, the good fortune to avoid injuries, and most of all passion for what you are doing.
I can't say your daughter has never had "the passion", but is it something she once had, or was she just having fun, and didn't need it to get to this point. If she truely has no passion for the sport, she may need to move on to discover something she is passionate about, and in that sense she will experience one of the most valuable lessons in life.......knowing when, where, or what to dedicate your energy to.