This is the hidden truth Paul began to explain in the previous verse. The Gentiles who receive Christ also receive the same things as the Jewish believers.
"In Christ Jesus" and similar expressions are metaphors that frequently occur in the New Testament letters. They express the strongest kind of relationship possible between Christ and those who believe in him.
# through the gospel
Possible meanings are 1) because of the gospel the Gentiles are fellow sharers in the promise or 2) because of the gospel the Gentiles are fellow heirs and members of the body and fellow sharers in the promise.