Well when I use term gable its the "end" side of your lean too I am referring as opposed to eave which you have said is ventilated. How do you think the heat is escaping thru the existing insulation
Venting the ends would not work or be possible due to the direction of the rafters.....
I am not sure it is so much that the heat is escaping, but that the roof is getting warm due to improper ventilation.