Is it OK to use a rectangular undereave vent or other style gable louver as the shutter for a gable-mount fan?

It is best to use the automatic opening/closing shutter designed for the gable-mount power fan.

There is a concern that using a soffit panel or undereave panel on the exhaust side of a gable mount power vent will be too restrictive for the application. Results may include ineffective attic ventilation (airflow) and possibly even premature motor failure. While the customer may not want to use the gable shutter that we have in our product line, there are other gable shutters and static gable louvers on the market that would be less restrictive so that effective attic ventilation could be accomplished while minimizing the potential for motor failure. Air Vent has not tested the desired soffit panel or undereave panel for compatibility with the power vent. The undereave panel very likely will reduce the CFM capability of the power vent. It's possible the undereave panel (because of its limited openings) could cause the motor to prematurely burn out. The shutter provided with the product is closed when the power vent is not running. The undereave panel desired will remain "open" to the weather at all times and that is not ideal weather protection.