If there is not a lot of blackness underneath the roof sheathing in the attic then the gable vents are doing their job. That means the attic is being ventilated adequately. Don't mess with something that is working.
Assuming this, clean the gable vents and blow in a minimum of 14 inch cellulose insulation across the attic floor. This will keep a naturally hot attic from radiating down through the ceiling in summer making the AC work harder, and help keep warm air from rising into the attic in winter. I think that's all you want.
Trying to get balanced heating and cooling with a one z