[Eric 951]

Don,

Will your garage be open to the roofline , or will there be a ceiling with space above up to the roof?

Unless paired with a supply fan (or intake louvers) on the opposite wall for a cross draft (as cashflyer posted), a stand-alone exhaust fan located in the gable is not worth it. You ideally want to create airflow and there are (2) efficient ways of doing so.
Intake air near the ground level and exhaust it through the roof(ex. windows or garage doors being open to draw air in) and roof-mounted turbine vents (the mushroom-looking vents), a true ridge vent, or powered exhaust fans mounted on the roof--all are designed to vent the hot air out just as a chimney operates(gravity ventilation).
Yes, this will also work on a limited basis with a wall-mounted exhaust fan to vent the hot air but is not nearly as efficient as a roof-mounted exhauster, and you have no means of moving the air to the single wall fan location--a lot of hot air will remain trapped.
If you are mounting in a wall for your exhaust, your intake should be on the opposite wall to create a cross-draft.
For your application and the added expense of installing a single powered wall exhaust fan, you would be better off going with a fixed gable vent instead of the powered fan, or one of the roof options mentioned, or a gable vent and a ridge vent.