Paul called it his gospel in Romans 2:16 KJV, Romans 16:25 KJV, 2 Timothy 2:8 KJV why shouldn't we? Paul was given the dispensation of the gospel (1 Corinthians 9:17 KJV) having received his gospel (1 Corinthians 15:1-4 KJV) by direct revelation of the risen Lord Jesus Christ (Galatians 1:11-12 KJV).Sometimes I do not understand why we say 'Paul's gospel'. Was Peter, etc. saved under Paul's Gospel? or do we consider that Peter. etc. was not saved BEFORE Paul came into the scene? There is no such thing as Paul's Gospel. It is the same problem I have when people tell me that it is the Revelation of John. There is no such thing as the revelation of John in the book of Revelation.
That which Peter preached was that which God hath spoken by the mouth of all his holy prophets since the world began (Acts 3:21 KJV) while Paul's gospel was kept secret since the world began (Romans 16:25-26 KJV) and Paul tells us why (1 Corinthians 2:6-8 KJV). Peter was not saved as we are having now received the atonement, but looks forward to salvation at the second coming of Christ (Acts 3:19-21 KJV, 1 Peter 1:7-13 KJV).