How could one "brought up at the feet of Gamaliel" ever have been "without the law once"?
He was once an infant (born of a Jewish mother/womb yet Roman father thus having the Latin name Paul), not knowing good/evil prior to being "brought up"/educated by Gamaliel.
Acts 22:2-3 And when they heard that he was addressing them in the Aramaic language, they became even more silent. And he said, “I am a Jewish man born in Tarsus in Cilicia, but brought up in this city at the feet of Gamaliel, educated according to the exactness of the law received from our fathers, being zealous for God, just as all of you are today.
Romans 7:7 What then shall we say? Is the law sin? May it never be! But I would not have known sin except through the law, for I would not have known covetousness if the law had not said, “Do not covet.”
Paul's "autobiography" summary is:
1. Born 1/2 Jew, 1/2 Roman/Gentile.
2. An infant not knowing right from wrong even in the simplest of commandments (don't covet).
3. Convictions of sin coming coming through learning the Law (and the traditions of Jews).
4. Weekly/yearly cleans through early adulthood.
5. Died to those traditions (released from the old like a widow is) and given eternal life through re-marriage to Christ.
I.e. Pretty much like the rest of us.
Galatians 1:11-12 For I make known to you, brothers, the gospel that has been proclaimed by me, that it is not of human origin. For neither did I receive it from man, nor was I taught it, but I received it through a revelation of Jesus Christ.
BTW, if Paul in the opening chapter of Galatians is making known the Gospel to his "brothers", are they Jewish brothers or Christian brothers?