Next switch the gun fire mode to safe if it is not already and begin to push the gearbox out of the receiver.

You may need to switch the selector back and forth between safe and semi auto to push out the gearbox. The gearbox should be able to be push slightly forward and then up out of the receiver.

You can grab the from the top and push the gearbox out from the bottom where the pistol grip was.

With the gearbox out the selector indicator on the right side of the gun should fall right out.