So, when Paul said circumcision is nothing, he meant that it did not mean anything to be saved or be part of God's kingdom. This is true for Jews and Gentiles in the context of salvation, not being chosen by people for a role or task.
Paul didn't write specifically to gentiles; he wrote to churches that undoubtedly had Jewish members because scripture plainly says so.. So when he wrote that circumcision is nothing, he meant that it is nothing in God's eyes. Circumcision no longer brings people into covenant relationship with God as it did from Abraham to Pentecost. Only spiritual circumcision makes one chosen, and that is accomplished only through faith.