Should I add another gable vent to my attic?
You need to consider airflow paths. Adding random vents is not always a good idea, as it can reduce flow from other parts of the system. Adding gable vents to a soffit/ridge vent system will likely reduce flow from the edges of the space. 

If you have good soffit ventilation, do not add more gable vents. Instead, add ridge venting. If the soffits are solid, gable vents could be a good idea.