No. A person can lose his gun rights even his he only received a slap on the wrist for his crime. All that matters is that the crime be punishable by more than a year in jail. Punishable, not punished.
Just like every other right, including even our most basic freedoms, this right can be taken away as a punishment for crime, as long as it is by due process of law and the punishment is not considered cruel and unusual by the courts. This idea is very solidly grounded in English commonlaw and our own traditions.