Yes, you need gear reduction for that speed. 3Nm is a lot of torque without a gearbox. 6RPM at 3Nm is not a lot of power, so you don't need a large motor.
Start with a small cheap motor such as a NEMA17 and a driver based on the DRV8825 or A4988 chips. That will get you started with learning about micro stepping. Power supply for these drivers should be about 12V with 1 or 2 amps current available