We are often given to forgiving the sins of those who "are on our side" and we often want to claim those "on the other side" who we agree with. Gamaliel was not stated to be among the believers in any Scripture but became considered a saint by some later tradition. Certainly his wisdom and holiness shine in the story told about him but there is no proof he was ever a disciple.
Another saint today is Anthony the Roman, who though born in Rome sided with the Orthodox during the Great Schism and ended up in Russia.
I think that we must be careful when we claim victory or tally up our side but in humility we must see the holiness in all who follow God, and believe, in His mercy, that all who seek Him with a sincere heart are rewarded with Heaven.
When they heard this, they became infuriated and wanted to put them to death. But a Pharisee in the Sanhedrin named Gamaliel, a teacher of the law, respected by all the people, stood up, ordered the men to be put outside for a short time, and said to them, “Fellow Israelites, be careful what you are about to do to these men. Some time ago, Theudas appeared, claiming to be someone important, and about four hundred men joined him, but he was killed, and all those who were loyal to him were disbanded and came to nothing. After him came Judas the Galilean at the time of the census. He also drew people after him, but he too perished and all who were loyal to him were scattered. So now I tell you, have nothing to do with these men, and let them go. For if this endeavor or this activity is of human origin, it will destroy itself. But if it comes from God, you will not be able to destroy them; you may even find yourselves fighting against God.” They were persuaded by him.
-- Acts 5:33-39