Gynecomastia is breast tissue developing. It is common for a males breasts to enlarge a bit early in puberty but it usually resolves quickly. Please see a doctor for a proper diagnosis. Don't just assume you have it.
What supplements are you considering, specifically? Seriously, most are a waste of your money. You need a well balanced diet, plenty of sleep and exercise to develop - NOTHING more.
To add, you are in puberty. Your body is making lots of testosterone. Testosterone helps build muscle mass. Synthetic hormones in some "supplements" can cause your testicles to shrink, liver problems, and sexual problems.
Just because you can buy this "stuff" it does not mean it is safe. Most "supplements" are unregulated.
Be safe, good luck.