This achievement is pretty self-explanatory. There's 8 different challenge types in the game. Each challenge type is represented by a gear mod category.
Destruction Frenzy (explosion icon)
Crash Bomb (bomb icon)
Shooting Gallery (cross hair icon)
Scrapyard Scramble (tether icon)
Wingsuit Course (wing icon)
Land Race (steering wheel icon)
Air Race (plane icon)
Sea Race (anchor icon)
Earn at least 3 gears in each of these challenge types for the achievement.