If you just need to remove the large gear, here's the time to do it. You can lift it off with a pair of pliers underneath. Note that this is attached to a plastic gear inside the gearbox and this is the usual failure point if your low speed attachment is not working but your mixer runs.
Next, undo the screws holding the gearbox onto the mixer body. There is one on either side of the mixer (only one shown). If your motor is dead, you may not need to do this.