Becoming a gymnast is not a realistic way of adding muscle mass, unless this epiphany strikes you at about the age of seven.

Even if you do decide to go the gymnast route, you'll still be lifting weights (though not until you've been at it for years, and you probably won't be making some newbie gym mistakes like prioritizing biceps), it'll just take longer to achieve results because you're mostly focused on acquiring the skill and technique to do actual gymnastics.

As for your other claim, body builders do not accidentally add too much muscle. It is a wide-spread fear among people who don't exercise at all, but it does not actually happen. You absolutely, positively, won't wake up one day and realize that you've accidentally put on too much muscle.

Accidentally putting on so much muscle as to be detrimental to your health would be akin to walking home from school or work and suddenly realizing you accidentally walked around planet earth and the soles of your feet have been completely worn down — you will not suddenly complete a journey that takes most people extreme dedication and hard work over many many years.

I didn't say go become a gymnasts. Anyone can do many of the conditioning routines gymnasts do to improve strength and technique. Push-ups, dips, crunches, v-ups, etc. are the most basic things most gymnasts do to strengthen their muscles and since most people are not going to be pushing boulders around I don't see a point in doing any kind of heavy lifting if you're exercising simply to reap the benefits of exercise.

Also, I didn't make a claim about accidentally adding too much muscle. I simply stated that the kind of muscles that weight-lifting builds is different from the kind of muscles you will build if you don't use weights. I've done both and simple routines that I can do at home exercise more muscle groups without isolating specific areas and overall I feel much better.