Red John is definitely Gale Bertram! There are many reasons which were already explained in many theories why Gale Bertram might be Red John. There is an additional clue that as far as I noticed here, has not been mentioned before. Gale Bertram has a an award in his office for bravery in the war.
Carter Peak, the victim that has been killed by Red John and of which Jane assumes that Red John might has made a mistake with him and might have left some trace that could reveal his identy (thus towlen morning his doctor and the bosco team were killed too) was a former soldier in the gulf war.
I am 100% sure you will see in season 6 amongst all this link between Bertram and Peaks revealed and you will see Gale Bertram is RED JOHN.