Can gynecomastia become less noticeable with exercise?

Good question.  Exercise can reduce fat and build muscle but it won't shrink breast tissue.  Thus, it depends on or you have true gynecomastia or just fatty breasts.  I recommend getting checked out by a board certified plastic surgeon to give you a more solid answer.

Your body continues to develop through adolescence, and most boys find that their gynecomastia corrects itself of its own. A boy with gynecomastia should first try to lose weight by going on a healthy diet and exercise regimen. This helps improve the chest contour. If after losing weight their chest is still not aesthetically pleasing, surgery can then be considered once another medical condition has been ruled out.

Thanks for your inquiry.  Exercise may decrease the amount of fatty tissue on the chest (and elsewhere on the body), but if there is glandular tissue it will not decrease with exercise.