"Gospel" means "good news".
What "good news" did Peter preach"?
At Pentecost (A Jewish Holy day), Peter preached a murder indictment to all the house of Israel (Acts 2:22-23 KJV, Acts 2:36 KJV), that they had murdered their Messiah, the Lord Jesus Christ, but that God hath raised Him up!
Acts 2:22 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:
Acts 2:23 Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain:
Acts 2:24 Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it.
Acts 2:25 For David speaketh concerning him, I foresaw the Lord always before my face, for he is on my right hand, that I should not be moved:
Acts 2:26 Therefore did my heart rejoice, and my tongue was glad; moreover also my flesh shall rest in hope:
Acts 2:27 Because thou wilt not leave my soul in hell, neither wilt thou suffer thine Holy One to see corruption.
Acts 2:28 Thou hast made known to me the ways of life; thou shalt make me full of joy with thy countenance.
Acts 2:29 Men and brethren, let me freely speak unto you of the patriarch David, that he is both dead and buried, and his sepulchre is with us unto this day.
Acts 2:30 Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne;
Acts 2:31 He seeing this before spake of the resurrection of Christ, that his soul was not left in hell, neither his flesh did see corruption.
Acts 2:32 This Jesus hath God raised up, whereof we all are witnesses.
Acts 2:33 Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear.
Acts 2:34 For David is not ascended into the heavens: but he saith himself, The LORD said unto my Lord, Sit thou on my right hand,
Acts 2:35 Until I make thy foes thy footstool.
Acts 2:36 Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ.
Notice, their reaction!
Acts 2:37 Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?
Peter told them what they must do for the remission of sins. They must REPENT (change their mind) about WHO Jesus Christ was and be identified (baptized) in water for the remission of sins.
Acts 2:38 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.
He did NOT preach Paul's gospel. Paul's gospel was a mystery at that time Romans 16:25-27 KJV, 1 Corinthians 2:6-8 KJV and since when does Paul tell us to DO anything to be saved?