I think a good place to look is John 20:19 On the evening of that day, the first day of the week, the doors being shut where the disciples were, for fear of the Jews, Jesus came and stood among them and said to them, “Peace be with you.” 20 When he had said this, he showed them his hands and his side. Then the disciples were glad when they saw the Lord. 21 Jesus said to them again, “Peace be with you. As the Father has sent me, even so I send you.” 22 And when he had said this, he breathed on them, and said to them, “Receive the Holy Spirit. 23 If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained.” 24 Now Thomas, one of the twelve, called the Twin, was not with them when Jesus came. 25 So the other disciples told him, “We have seen the Lord.” But he said to them, “Unless I see in his hands the print of the nails, and place my finger in the mark of the nails, and place my hand in his side, I will not believe.”
26 Eight days later, his disciples were again in the house, and Thomas was with them. The doors were shut, but Jesus came and stood among them, and said, “Peace be with you.” 27 Then he said to Thomas, “Put your finger here, and see my hands; and put out your hand, and place it in my side; do not be faithless, but believing.” 28 Thomas answered him, “My Lord and my God!” 29 Jesus said to him, “Have you believed because you have seen me? Blessed are those who have not seen and yet believe.”
30 Now Jesus did many other signs in the presence of the disciples, which are not written in this book; 31 but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in his name.
This passage is so nice for many reasons. It starts off with the people who got to see Jesus while He was on earth, it turns into the doubting Thomas story where he needs “proof” that Jesus came back. It turns into Jesus leaving us with the blessing for those who believe in Jesus without demanding “proof”. I put proof in quotes because there are many ways people prove to themself these things are true, eg fulfilled prophecy, miracles, etc. In a sense proof is not always a guarantee of “seeing is believing”, in fact there were many people who saw Jesus do many miracles right in front of them and they still didnt believe!
The chapter ends with the statement that this book wasnt intended to be an exhaustive 25 volume encyclopedia type book. It gave enough info for anyone to believe in Jesus and know why He did what He did.
As for the “whole” Gospel claim.
-Every Sacrament is recorded in Scripture,
-we have 4 books devoted to the life of Jesus,
-Acts of the Apostles showing people and converting them.
-early Christian writings
to name a few.
Also we need to remember that you cant understand everything using just a book, the Bible is one of billions of books out there, what made you pick it out of that ocean of books? You need people who have learned from earlier Christians and had that information passed on to them, ie traditions. Another thing is that the Bible cant Baptize you only a human can. One thing that most protestants dont realize is that Jesus established a Church, He didnt write a book. That doesnt mean the Bible is bad it means that Scripture and Tradition are both authorities and go hand in hand.