With the screws removed take a long thin screw driver and insert it into the back hole of the gearbox. This will prevent parts from shooting out when removing the shell.
You can now us your finger nails and separate both sides of of the gearbox shell and pull the top half of the gearbox off.
Next remove the spring and spring guide by lift the spring guide up and out of the gearbox. Then take out the cylinder and connected parts to have access to the gears.