Yes, the gable vents (a type of exhaust vent) should be closed off whenever a ridge vent (which is also a type of exhaust vent) is installed because vents installed at the roof’s edge or in the overhang should supply the intake air needed by the ridge vents. Air should flow in through the intake vents evenly along the roofline and exhaust out the peak. Any vents in place between the ridge vents and the intake vents may interrupt or short-circuit that flow of air along the roofline. The gable vents will end up becoming intake for the ridge vent — an undesirable situation that could lead to weather infiltration through the gable vents and also could prevent the attic from being properly ventilated. The same is true with mixing wind turbines or roof louvers with ridge vents.