When using LS Grease No.0 / No.2 on airsoft guns, how should they be used? Is it correct to use No.0 for the piston (and cylinder gap) and No.2 for the gear section?

Your understanding is correct.

LS Bell Hammer Grease No.0: A soft, low-viscosity type suitable for sliding and contact areas inside pistons and cylinders.

LS Bell Hammer Grease No.2: A medium-viscosity, high-load type ideal for rotating parts under heavy stress, such as gear components.