They had just prayed for boldness in Chapter Four and now they were persecuted. Peters response is short and hit all the right buttons that would make them angry -- you killed him, God raised him, he sits exalted, and repent and be forgiven. That message to that audience is a "0" on a scale of 1-10 in terms of popularity. The Apostles stand fearlessly before the same crowd that crucified Jesus and say, "If it is right to obey men rather than God, you decide. We'll obey God."
Gamaliel's speech was persuasive. He cited two historical figures to say that if it is not from God, then it won't last. If it is from God and you oppose it, you'll be opposing God. So, leave them alone. News for them. . . we're still here!