For a dc gear motor, it is including two parts, the driving motor and the gearbox.
Dc gear motor means the gear motor is working under direct current voltage.
For the driving motor part, it could be either brushed dc motor or brushless dc gear motor.
The brushed dc motor could be carbon brush or electric brush.
The voltage of the dc motor determine the voltage of the dc gear motor.
The size and the quantity of the wire determine the driving motor speed.
For two different dc motor, even they have the same speed, they might use different wires.
For the gearbox part, it could be spur gearbox, planetary gearbox and worm gearbox etc.
The size of the gearbox, the gear ratios we choose, or the materials of the gears, all these factors might influence the performance of the dc gear motor.
So a dc gear motor is using a gearbox to reduce the speed of the driving motor, however produce higher torque to move the object.