Possible meanings are that the Galatians have become God's children 1) by believing God's promise or 2) because God worked miracles to fulfill his promises to Abraham, first by giving Abraham a son and then by making the Galatians children of Abraham and thus sons of God.
# according to the flesh
This refers to Abraham's becoming Ishmael's father by taking Hagar as a wife. AT: "by means of human action" or "because of what people did" (See: [[rc://en/ta/man/translate/figs-metaphor]])