Sorry but have to disagree with your point here. Read a little further in Acts 15:
Act 15:13 And after they had become silent, James answered, saying, "Men and brethren, listen to me:
Act 15:14 Simon has declared how God at the first visited the Gentiles to take out of them a people for His name.
Act 15:15 And with this the words of the prophets agree, just as it is written:
Act 15:16 'AFTER THIS I WILL RETURN AND WILL REBUILD THE TABERNACLE OF DAVID, WHICH HAS FALLEN DOWN; I WILL REBUILD ITS RUINS, AND I WILL SET IT UP;
Act 15:17 SO THAT THE REST OF MANKIND MAY SEEK THE LORD, EVEN ALL THE GENTILES WHO ARE CALLED BY MY NAME, SAYS THE LORD WHO DOES ALL THESE THINGS.'
Act 15:18 "Known to God from eternity are all His works.
James made the determination that Peter spoke truly, but he also verified that what Peter said was completely consistent with scripture. They didn't just allow someone to say something without having a scriptural basis. In this case he referenced Amos but there were many scriptures that foretold that gentiles would be granted the holy spirit of the Lord. The bottom line is that Peter wasn't doing anything on tradition or his own authority but instead looked at scripture and determined whether what was happening was scriptural or not. It was in this case.
No one has even remotely suggested that gentiles aren't called to be part of the plan of salvation. This is a straw man. I think you were trying to prove that St Peter's assertion that gentiles need not be circumcised was based in Scripture... your citation above doesn't discuss this. Can you try again?