As usual you are confused. Our blessings do not come through a covenant which the LORD made with the physical descendants of Abraham, Isaac and Jacob but instead by the Last Will and Testament of the Lord Jesus, spoken of here. The following verse speaks of the Lord Jesus' Last Will and Testament and in this case it speaks of something associated with wills--an inheritance:
"And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance" (Heb.9:15).
A person becomes a "heir" when he receives an inheritance which flows from a Last Will and Testament. Therefore, in the following verse Paul is using the term "gospel" as a synonym for the Lord Jesus' Last Will and Testament.
"That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel" (Eph.3:6).
Sir Robert Anderson writes that
"Our spiritual and eternal blessings do not depend on a covenant made with us, but upon a testament under which we are beneficiaries" (Anderson,
Types in Hebrews [Grand Rapids: Kregel, 1978], p.56).
How can you possibly be so confused, not even knowing that our blessings flow from the Lord Jesus' Last Will and Testament and not from a covenant made with the physical descendants of abraham, Isaac and Jacob?