A gearbox assembly (10) including a system of stacked planetary gearsets; a clutch device (16) including: an input clutch shaft (86) and an output clutch shaft (90); a large clutch wheel (94) and a small clutch wheel (96) mounted idle on said output clutch shaft (90); a clutch coupling device (98) for coupling the small (96) and large wheels (94) with the output clutch shaft (90); an output shaft (13) including a small output wheel (114) meshing with the large clutch wheel (94) and a large output wheel (116) meshing with the small clutch wheel. The assembly also includes a decoupling device (118) between the output shaft (13) and the large output wheel (116).
