The resurrection of Christ is found in all four gospels, because the life and death of Christ are meaningless apart from the resurrection.  If Jesus is still in the grave then, as Paul says in 1 Cor 15 “we of all people are the most to be pitied.”  But he’s not still dead.  He has risen, which is what John 20 is all about.  But John 20 is not just here to tell us that Jesus has risen from the dead, but to convey a particular message and to make a particular point.