Dispensationalism teaches that Jesus preached to the Jews and Paul to the Gentiles.
They also teach that the message was different.
They teach that Jesus preached “law” and Paul preached “grace”, the conclusion being that they preached different Gospels.
As you know that is not true, the Gospel preached by both was/is one and the same.
Adam Clarke Commentary:
The whole doctrine of Jesus Christ, comprised in the history of his incarnation, preaching, miracles, sufferings, death, resurrection, ascension, and the mission of the Holy Spirit, by which salvation was procured for a lost world, is expressed by the word euaggelion (gospel) … the sacred writers use it with a variety of epithets which it may be necessary to mention.
Strong’s Greek Dictionary:
a good message, i.e. the gospel:–gospel.
1st, It is sometimes termed, The Gospel of God concerning his Son. Romans 1:1,3.
2dly, The Gospel of the Son of God. Romans 1:9.
3dly, The Gospel of the kingdom of God. Matthew 4:23; 9:35; 24:14; Mark 1:14.
4thly, Sometimes it is simply called THE GOSPEL. Mark 13:10; Mark 16:15.
5thly, The word or doctrine (logos) of the Gospel. Acts 15:7.
6thly, The Gospel of peace. Ephesians 6:15.
7thly, The Gospel of glory . . . 1 Timothy 1:11.
8thly, The Gospel of salvation . . . Ephesians 1:13.
In 1Corinthians 9:23, it means the blessings and privileges promised in the New Testament.
It means the public profession of the doctrine taught by Christ, Mark 8:35; 10:29; 2 Timothy 1:8; Philemon 1:13.
John 1:17 For the law was given by Moses, but grace and truth came by Jesus Christ.
Galatians 3:6-14 Even as Abraham believed God, and it was accounted to him for righteousness. Know ye therefore that they which are of faith, the same are the children of Abraham. 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. So then they which be of faith are blessed with faithful Abraham. For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them. But that no man is justified by the law in the sight of God, it is evident: for, The just shall live by faith. And the law is not of faith: but, The man that doeth them shall live in them. Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree: That the blessing of Abraham might come on the Gentiles through Jesus Christ; that we might receive the promise of the Spirit through faith.