en_tn_lite_do_not_use/eph/03/06.md

the Gentiles are fellow heirs ... through the gospel

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.

fellow members of the body

The church is often referred to as the body of Christ. (See: rc://en/ta/man/jit/figs-metaphor)

in Christ Jesus

"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.