Gal 3:8, Acts 3:25, Gen 18:18, Gen 22:18, Gal 3:16, Matthew 28:18-19
The gospel that God preached to Abraham is that Christ will come as Abraham's descendant and will bless all peoples. (Gen 12:3, Gen 18:18, Gen 22:18, Gal 3:8, Acts 3:25)
Gen 12:3 And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
Gen 18:18 Seeing that Abraham shall surely become a great and mighty nation, and all the nations of the earth shall be blessed in him?
Gen 22:18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.
Gal 3:8 And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
Acts 3:25 Ye are the children of the prophets, and of the covenant which God made with our fathers, saying unto Abraham, And in thy seed shall all the kindreds of the earth be blessed.
The descendant that God promised Abraham is Jesus, the Christ. (Gal 3:16)
Gal 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
We must proclaim to all nations that Jesus is the Christ, so that they may be blessed. (Mt 28:18-19)
Mt 28:18 And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. 19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: