Peter, John and James preach the same Gospel as Paul. Christ crucified, died and Risen from the dead. Same Gospel and same Grace.
...but they do not. Recall James "faith without works is dead". Peter anticipates the activity of the Adversary in their suffering. (1 Peter. 5:8,9). 2nd Peter anticipates the advanced issues of resisting the seduction of the Satanic Policy of Evil (2 Pet. 3:17). John's 3 Books focus on the tests necessary for detection of error and discerning for truth, and overcoming the wicked one (1 John 2:14). Jude anticipates the end result of God's judgement and the glory of Christ's reign of Revelation. (Jude 22-25). And Revelation speaks for itself. Rev. 22:18-19 anticipates that there is nothing more to link to- it is all "done". God has revealed ALL He needs to about the future and His purpose to glorify Himself through Jesus Christ in it.
Each Book links itself to the following Book. Beginning with Hebrews, written TO THE HEBREWS, and is all about "these last days" (Heb. 1:2). And so on through Revelation.
It is for these reasons that I believe that Hebrews is written as instruction and doctrine for Daniel's 70th week.
As far as "the same Gospel", please read Rom. 2:16. Paul calls it "my gospel", not "our gospel".