Is installing a gable fan worth the investment?
I can tell you from experience in my house  a slate roofed colonial, it will make a difference. Even with the fan mounted in an attic window that is that is 3 inches off the floor of my walk-up attic the temperature drop in the attic was 30 degrees.Most fans will have a spec sheet telling how many square inches of ventilation opening they require. It is important to remember that you are calculating square inches of unimpeded opening so subtract for louvers and other obstructions. In my case I have a large peak where hot air tends to get trapped. If we have several days of very sunny and hot weather I place a small table fan on the attic floor. With the small fan pointing up it circulates the air to remove the hottest air trapped at the ridge.