How much attic ventilation is needed?
Many building codes require at least 1/300 of the attic area in free openings for ventilation when cross ventilated, which it sounds like you have. If not cross ventilated, you need 1/150 of the area. Be sure your soffit vents are not obstructed by insulation. If the ridge and soffit vents meet this requirement without the gable ends, you might consider closing off the gable ends.

My guess is they were put there for a reason and you will not have adequate free area without them. I don't think you get extra credit for turbine vents though they can move more air because they don't move much air in still conditions. You should avoid mechanical ventilation if at all possible IMO.