The unfortunate truth is that you cannot target where the fat loss will be. It is perfectly normal for men to have fatty deposits in the breast area, for this to be one of the first places where fat is deposited, and for this to be one of the last places where fat diminishes with weight loss.
There are men who are quite thin and actually underwight who have gynecomastia.
Weight loss and/or exercise will not usually reduce gynecomastia to a signicant degree unless you are grossly obese.