Did Peter understand God's move from Israel to the Gentiles with the gospel of grace?
No. For the most part, Peter had no comprehension of God's move away from Israel and the covenant promises to the Gentiles with the gospel of grace. There was a difference of opinion, and Peter later acknowledged that the things written by Paul are hard to be understood.
Les explains that the theme of Galatians is that we are not under law but under grace, a statement that came first and foremost from the Apostle Paul. He notes that even Peter for the most part had no comprehension of this great move of God to come away from Israel and the covenant promises and go out to the whole world, Jew or Gentile, with the gospel of the grace of God. Les says this was not known until Paul, and he saw a great difference between Peter's message and Paul's, calling it daylight and dark. Later in the chapter, Les points to a difference of opinion, and Peter had a hard time, writing in his second epistle that the things written by Paul are hard to be understood.