Is it alright for a guy to do gymnastics?

Absolutely, yes. And gymnastics is one of the only sports that works every single muscle in the body, so it keeps you fit better than most sports. You develop abs in the gym due to the fact that gym. requires a great deal of core body work, as well as working the extremities better/more than almost any other sport (hands, wrists, forearms, ankles, etc). It is only possible for gymnastics to stunt the growth if you have been training heavily from a very young age (I suggest you look up the heigths of famous male gymnasts, but this is not a foolproof way to tell because there are so many other factors- genetics,etc that come into play). There is no set amount of how many times to go to the gym, the more you go, the better you get at it, and you usually have to spend a significant amount of time and energy in order to be good at it because it requires so much more than other sports, such as strength, flexibility, balance, cardio etc. Gymnastics is a strength-to-weight ratio sport, which is the main difference between it and contact sports (baseball, football,etc). It is much more than enough to replace working out. You are not likely to get injured if you are conditioned properly and learn skills in order, not attempting skills you aren't prepared for. I would suggest it because it is so demanding that any skills you develop in the gym will make you better at any other sport you choose to pursue! For example, I am a former gymnast, and I went wakeboarding with friends, and the first time I went I was able to do flips and things my friends were all working on! And nothing is more fun than flying through the air, be it over water or land! However, your friend might be surprised how demanding the sport is, although it's well worth the effort!