In my point of view its similar to weightlifting. Most of the athletes (sprinters, hockey players, american football players, MMA fighters) put some principles of weightlifting into their training (they most often use power clean from hang) to build some "abilities" they need (in this case explosivness, whole body explosive projection and acceleration). They dont use whole weightlifting routines, they do not train 6x per week front squats and they not learn how to perform full clean and jerk. But they take the principles of weightlifting that are useful for them in this case same partional lifts (from hang, from blocks) and put them into they weight training.
And same it might be with gymnastics. Just to be sure we are talking about gymnastics on rings, on floor, on paralettes etc. with background in competitive gymnastics not about balancing on fitball or doin ostrich emu pose on bossu - that is not gymnastics. I am not advocate of anything called functional training, I am tryin to say that if something build strength that can be told "functional" that it is gymnastics.
So why athletes dont include gymnastics in their weight training if it is so good? Answer is just because they dont know about its benefits. As in the example with weightlifting its nonsense to do same things as professional gymnasts. But we can take some principles from their training that can be included in everyones training because of their power. As with weightlifting athletes take power clean from hang, from blocks, power snatch from hang, jumps etc., they may to learn how to hold some basic positions on rings, paralletes etc. such as L-sit, handstand, HSPU, back lever, iron cross etc. And by the way of learning them, gymnastics give them tremendous functional strength of back, abs and biceps. And strengthened their tendonds.
So while principles of weightlifting and powerlifting give athletes strength and explosivness of performance muscles: delts, pecs, triceps, hamstrings, calves and partialy quads - learning of gymnastics is here mostly for backs, core and biceps and tendonds - parts of body using to hold better body tension - where better ability to hold body in high tension means better background for doing weight lifts. Thats why if someone start with gymnastics - other lifts such as squat and deadlift goes up.
The real athletes these days dont do squats 5 sets per 10 reps in 60% zone, they do 8 sets of triples of squats with maximal force and velocity with weight near they maximum - this type of squating build functional strength (by sarcomere hypertrophy and high threshold motor units recruitment) while 5x10 build non functional strength (by sarcoplasmic hypertrophy). And same: doin bent over rows and biceps curls build nonfunctional strength of these muscles while microsocilating and functional and overcoming isometric in gymnastics exercices build functional strength in this muscles. And same it is with core and stability. Balancing on bossu do shit but balancing on one hand in handstand is royal way to control every muscle in your body to hold a balance.
It is not about doing just gymnastics - its about combination of weightlifting/powerlifting and gymnastics and sprints (for examples) to have overall functional physique.
There is video of me not doing just gymnastics:
and here is one of the best MMA fighters od all times George Saint Pierre and his very wise training of combination of weightlifting and gymnastics:)