Should I keep attic gable windows open for ventilation?
Attics should remain ventilated, even in winter. Your gable windows should add important circulation to the airflow path. Ideally air is scavenged from all parts of the attic.

Typically there are louvered vents on gables, not windows. You'll need to look into some sort of add-on louver or awning, or replace the windows with something more appropriate.