gcm is gram * centimeters. The stall torque is listed as 340 gcm. This means that if you stuck a pulley with a radius of 1 cm on the shaft, wrapped a string around it, and tied it to some weights, it would fail to lift more than 340 grams.
Your strategy of 4 AA batteries and a switch will probably work. If the system involved digital electronics, you'd want to add a diode in parallel with the motor to prevent voltage spikes when you opened the switch. (Google "flyback diode.") You'd also want a capacitor in parallel with the motor to filter out high-frequency electrical noise, which would scramble the rest of your circuitry. But for a small motor and a mechanical switch, you'll be fine.