Huh. I wonder if this conversation would go differently in the roofing forum. I'm no roofer, but the way I understand it, gable vents are usually installed at opposite ends of the house. They don't cool the attic by convection, but by cross ventilation, one supplying air to the other. Not saying it's perfect, maybe a flawed theory, but that's the intent. If anything, the roof vents (Mucluk didn't say ridge, maybe turtle?) are superfluous, in theory.