The Epistles are for people who already know the Gospel in established Christian communities, so if the Epistles are not the Gospel then what is the Gospel? Should Matthew, Mark, Luke and John be used exclusively to tell the Gospel to non-believers?
Perhaps you could say we have theological license to use what we want to or it could be said that doctrinally meaty messages are irresponsible for non-believers as they are not ready. This begs the question what is the purpose of the Gospel? Should it "fix" ones theology or should it be more focused on introducing someone to Jesus?
If you make the entire 4 gospels out to be the gospel message then you will have problems, such as btwn the synoptics (which do not even mention the new birth), but the Epistles explain the Gospel which Christ commanded to be preached and which we see concisely in Acts, which book you left out.
And
this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come. (Matthew 24:14)
And he said unto them, Go ye into all the world, and
preach the gospel to every creature. He that believeth and is baptized shall be saved; but he that believeth not shall be damned. (Mark 16:15-16)
And said unto them, Thus
it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day: And that
repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem. And ye are witnesses of these things. And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with
power from on high. (Luke 24:46-49)
The basic gospel in essence, salvation by faith in the substantiated death, burial and resurrection of the prophesied Christ of Scripture.
Moreover, brethren,
I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
By which also ye are saved, if ye keep in memory [living faith] what I preached unto you, unless ye have believed in vain. For I delivered unto you first of all that which I also received, how that
Christ died for our sins according to the scriptures; And that he was buried, and that he rose again the third day according to the scriptures: And that he was seen of Cephas, then of the twelve: After that, he was seen of above five hundred brethren at once; of whom the greater part remain unto this present, but some are fallen asleep. (1 Corinthians 15:1-6)
By which souls are translated into the kingdom of God (which is spiritual, and while the literal physical establishment, spurned by the 1st c. Jews, awaits the Lord's return).
Jesus answered,
My kingdom is not of this world: if my kingdom were of this world, then would my servants fight, that I should not be delivered to the Jews: but now is my kingdom not from hence. (John 18:36)
Who hath delivered us from the power of darkness, and
hath translated us into the kingdom of his dear Son: (Colossians 1:13)
"This gospel of the kingdom" was preached in Acts:
But when they believed
Philip preaching the things concerning the kingdom of God, and the name of Jesus Christ, they were baptized, both men and women. (Acts 8:12)
And now, behold, I know that ye all, among whom
I have gone preaching the kingdom of God, shall see my face no more. (Acts 20:25)
Preaching the kingdom of God, and teaching those things which concern the Lord Jesus Christ, with all confidence, no man forbidding him. (Acts 28:31)
Beginning at Jerusalem:
But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judæa, and all ye that
dwell at Jerusalem, be this known unto you, and hearken to my words: (Acts 2:14)
Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know: Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have
crucified and slain: Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it. (Acts 2:22-24)
For David is not ascended into the heavens: but he saith himself, The Lord said unto my Lord,
Sit thou on my right hand, Until I make thy foes thy footstool. Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ. (Acts 2:34-36)
Then Peter said unto them,
Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call. (Acts 2:38-39)
Likewise Paul primarily first to the Jews:
Men and brethren,
children of the stock of Abraham, and whosoever among you feareth God, to you is the word of this salvation sent. (Acts 13:26)
Men and brethren, children of the stock of Abraham, and whosoever among you feareth God, to you is the word of this salvation sent. For they that dwell at Jerusalem, and their rulers, because they knew him not, nor yet the voices of the prophets which are read every sabbath day, they have fulfilled them in condemning him.
And though they found no cause of death in him, yet desired they Pilate that he should be slain. And when they had fulfilled all that was written of him, they took him down from the tree, and laid him in a sepulchre.
But God raised him from the dead: And he was seen many days of them which came up with him from Galilee to Jerusalem, who are his witnesses unto the people. And we declare unto you glad tidings, how that the promise which was made unto the fathers, God hath fulfilled the same unto us their children, in that he hath raised up Jesus again; as it is also written in the second psalm, Thou art my Son, this day have I begotten thee. (Acts 13:26-33)
Be it known unto you therefore, men and brethren, that
through this man is preached unto you the forgiveness of sins: And by him all that believe are justified from all things, from which ye could not be justified by the law of Moses. (Acts 13:38-39)
To the Gentiles:
Then Peter opened his mouth, and said, Of a truth I perceive that God is no respecter of persons: But
in every nation he that feareth him, and worketh righteousness, is accepted with him. The word which God sent unto the children of Israel,
preaching peace by Jesus Christ: (he is Lord of all That word, I say, ye know, which was published throughout all Judæa, and began from Galilee, after the baptism which John preached; How God anointed Jesus of Nazareth with the Holy Ghost and with power: who went about doing good, and healing all that were oppressed of the devil; for God was with him. (Acts 10:34-38)
And we are witnesses of all things which he did both in the land of the Jews, and in Jerusalem;
whom they slew and hanged on a tree: Him God raised up the third day, and shewed him openly; Not to all the people, but unto witnesses chosen before of God, even to us, who did eat and drink with him after he rose from the dead. (Acts 10:39-41)
And he commanded us to preach unto the people, and to testify that it is he
which was ordained of God to be the Judge of quick and dead. To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins. While Peter yet spake these words, t
he Holy Ghost fell on all them which heard the word. (Acts 10:42-44)
Then answered Peter, Can any man forbid water, that these should not be baptized, which
have received the Holy Ghost as well as we? (Acts 10:46-47)
Salvation by grace thru effectual heart-purifying justifying faith:
And when there had been much disputing, Peter rose up, and said unto them, Men and brethren, ye know how that a good while ago God made choice among us, that the Gentiles by my mouth should
hear the word of the gospel, and believe. And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as he did unto us; And put no difference between us and them,
purifying their hearts by faith. Now therefore why tempt ye God, to put a yoke upon the neck of the disciples, which neither our fathers nor we were able to bear? (Acts 15:7-10)
What the gospel accounts mean and fully entails (such as the "one new man") are provided in Acts and the epistles, being interpretive of the gospels, like as they and Acts and the epistles are interpretive of the OT.
Thus the most ancient and only wholly inspired substantive authoritative record of what the NT church believed is Scripture.