The matter at hand was one of being backslidden......not of Peter being ignorant or oblivious of the redemption of the Gentiles.
"Backslidden" may not be the fairest term when applied to Peter. Had you been a strict Torah-observant Jew who had been taught to keep his distance from "defiling" Gentiles because of what happened in the time of Ezra-Nehemiah, you would have had a very hard time accepting the fact that the Gentile "dogs" were suddenly being offered salvation. Do you recall what Jesus said to the Syrophoenician woman?
God had to use some strong persuasion to send Peter to the home of Cornelius. And even though Peter witnessed firsthand that God gave the gift of the Holy Spirit to all those Gentiles, and even though Peter had them all baptized, he would still have had some reservations. So when the Judaizers managed to persuade Peter to keep his distance from the Gentile believers, he went along with their nonsense. That Paul had to rebuke him for inconsistency meant that Peter was still getting used to the idea of Gentiles within the Church. The revelation of the "mystery" of the Church (Jew and Gentile in one Body) was given to Paul, not to Peter. Peter was from Galilee while Paul was from Tarsus. That too had an impact on their views of Gentiles.
While this really has nothing to do with Pentecostalism, their doctrine of being baptized "in the Spirit" also creates two-tiered Christianity. Those filled with the Spirit speak in tongues, and those not filled with the Spirit do not. So who is better?