This achievement will unlock after completing your 15th Championship Series in the career. You do not need to earn a gold medal on any race, although if you are following the road map you should have gold medals on all of your races as you move through the game. If you complete the leagues and championships in order, this achievement will unlock after completing the following Championship Series:
Modern Muscle Car (Sport League, 8 races)
American Muscle Revival (Sport League, 8 races)
Modern Grand Touring (Grand Touring League, 10 races)
Middle Grand Touring (Grand Touring League, 10 races)
Early Grand Touring (Grand Touring League, 10 races)
Note that you do not need to complete the bonus races included with the Championship Series for this achievement to unlock.