I'm seeing so many posts by teens who say they can't tell the parents because they will react negatively. I've just got to say this: Teens, your chances of getting through this with a positive outcome are much greater if you have the understanding of your parents. This takes open and honest communication with them. They are people, too, and they may have a hard time understanding at first. Don't keep everything from them for months or years and then get mad at them because they don't understand when you unleash all of it at once in a tidal wave of emotion.
That's a self-fulfilling prophecy - You say they won't understand so you hide it, then you drop this emotion-filled bomb that they don't immediately understand, then you say, "See, I told you so - They didn't understand." and you never say anything else to them about it.
You have got to keep the communication lines open with them with the little things. You may first say that you've noticed your chest is getting bigger and it's concerning you. If they respond that it's your weight, then try this: Say "Maybe you're right. Thanks. If I do some research on weight-loss in the chest area, would you look at it with me and help me decide the best plan?" In other words, let them find out about gynecomastia WITH you.
Also, If you have living grandparents who are close to you then they can be a tremendous asset for you in influencing your parents to get help for you, but be careful that it's not perceived that you're pitting one against the other.
The point of this long post is that you don't have to go through this alone. Ultimately, the pros of a parent's support far outweigh the cons of risking their not understanding. The longer you wait to start talking about your concerns with them, the greater your challenge will be in gaining their understanding. That's true not only with gynecomastia, but with everything else, too.
Just so you'll know: I'm over 40, father of 2, married to a very attractive woman, and didn't discuss this with my mother until last month. She said she had been concerned when I was a teen. She never talked to me because she thought I was OK with it since I never talked about it.