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.

At 14, you have
limited options to change the shape of your breasts. Exercising the chest wall muscles with
weights can add bulk and provide added definition to the chest. If you are overweight, losing fatty tissue
can also improve the appearance of the chest.
However, glandular breast tissue cannot be lost or altered though diet
or exercise; this can only be addressed through a surgical procedure, like the
male breast reduction. Liposuction can
be used to sculpt the chest, and remove excess fatty tissue. Once your breasts have fully developed, around
the age of 18, an experienced plastic surgeon can determine the best course of
treatment to achieve a flat, smooth chest.