If you have a continuous ridge vent across the roof and good soffit
venting, there is no reason you can't eliminate the gable vent. In
fact, there is debate over whether having gable vents with ridge vents
actually makes things worse. Some are of the opinion that the gable
vents allow a short circuting effect, where air enters from the gable
and exits throught the ridge vent, rather than allowing air to
circulate all the way up from the soffit vents to better cool the
attic. Plus, if you calculate the square footage of the gable vent, it
should be small compared to the area of the ridge vent. And finally,
you have a power vent as backup, with of course, the same potential
short circuiting issue. I'd make sure the soffits are sufficient for
the rating of the fan to avoid sucking conditioned air out of the house.