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?
It wasn't about whether gentiles needed to be circumcised or not. It was about whether or not gentiles needed to be circumcised to be saved.
Act 15:1 And certain men came down from Judea and taught the brethren, "Unless you are circumcised according to the custom of Moses, you cannot be saved."
The answer was obvious to all after hearing testimony that of course gentiles could be saved without being circumcised. They then used scripture to verify that GENTILES were to be saved. They interpreted this with the obvious meaning...that God promised in the old testament that uncircumcised gentiles would be saved.