How do I train my large adult dog to not behave aggressively towards some other dogs when walked on lead ?

We got this dog free out of the paper when he was aged about 18 months from a family that were moving to a unit. Now aged 6 he is great family dog. Very good with the children. I have no complaints except for when we walk him. I am not sure of his breeding but he weighs 40 kilos or so and is very strong.He displays very agressive behaviour towards other dogs when I walk him and goes beserk and is quite difficult to hold back. I use a gentle leader collar on him and sometimes a check chain which is harder to hold. Even though he cant breathe he will still do all he can to get to the other dog. It is very embarring and I am sure a little scary for other dog owners.If I am walking him and another dog owner has their dog off lead I have to call out to them to please put their dog on a lead as I am fearful that if their dog approached they would be killed. At home his obedience is good eg sit stay rollover etc . but around some dogs he just wont listen to me.Please provide training ideas