Yes, any simple air block material like plastic, OSB or plywood can be used to block air flow into the attic through the gable vents.

The minimum recommended attic ventilation is 1.5% of your attic floor space. If you have a 1000 square foot attic floor, then the ventilation should meet or exceed 15 square feet of vent space when you add the soffit vent area + ridge vent area + gable vent area. If the total of those areas are 15 square feet or more then the minimum attic ventilation is met.

The reason the gable vents need to be covered is to promote the convective air current in the attic with cooler air entering low on the eaves or soffit, and then rising to the highest part of the attic and exhausting or venting out the highest part of the attic. The lower or soffit eave part of the attic can be under-vented if the intake air comes through the gable vents and exhausts out the higher ridge OR pan vents. You must choose only one system to use; combining pan and ridge vents will short circuit the system and starve the roof of adequate ventilation.

A properly vented attic creates a strong convective air flow from the cooler, lower soffit intake vents and rises as that air becomes hotter in the attic and rises to escape out the ridge vent OR high pan vents. Mother Nature thus creates a powerful and effective attic exhaust flow.