A gear motor assembly includes a casing having a through hole, a motor fixed to the casing, an output shaft, a gear fixed to the output shaft and driven by the motor, and a substantially tubular bearing fixed in the through hole of the casing. The gear has a central hub which has an axial end surface, and a shaft portion extending from a central portion of the axial end surface of the central hub. The shaft portion is sleeved in the bearing. A slot is formed in the outer surface of the shaft portion and is filled with lubricant. The slot extends from a free end of the shaft portion to a joint between the outer surface of the shaft portion and the axial end surface of the central hub.