A gas spring-powered fastener driver includes a cylinder, a moveable piston positioned within the cylinder, a driver blade attached to the piston and movable between a ready position and a driven position, a lifter engageable with the driver blade to return the driver blade from the driven position to the ready position, and a multi-stage planetary transmission including an output shaft operatively coupled to the lifter to provide torque thereto, a first bearing supporting a first portion of the output shaft for rotation, a second bearing supporting a second portion of the output shaft for rotation, and first and second planetary stages. The fastener driver also includes a housing having a cylinder support portion and a transmission housing portion in which the first and second bearings are received. The cylinder support portion and the transmission housing portion are integrally formed as a single piece.