Cool air its drawn from soffit vents by a power fan or ridge vents.
Gable vent works by creating a draft thru the attic by being installed on both ends of the house...
I prefer soffit vents in combination with ridge vents or power (attic) fan.
You can also add an attic fan (as close as possible to the ridge) now to draw more air from the gable vents but still it will not be effective as a ridge cap.
Close the gable vents off and install a power fan in combination with soffit vents
or ridge vent instead.