The Maharsha explains the conclusion of the Gemora: Tzadik v'ra lo is by a tzadik sh'eino gamur because he gets punished in this world on account of his few sins in order that he will be rewarded in the World to Come. On the flipside, a rasha sh'eino gamur is rewarded in this world in order for him to receive his payment for his few good deeds, and this way, he will not merit a portion in the World to Come.