I’ve read and heard that John’s gospel is 90% unique.
From New Advent:
St. John the Evangelist
newadvent.org/cathen/08492a.htm
"John was the son of Zebedee and Salome, and the brother of James the Greater.
In the Gospels the two brothers are often called after their father “the sons of Zebedee” and received from Christ the honourable title of Boanerges, i.e. “sons of thunder” (Mark 3:17).
Originally they were fishermen and fished with their father in the Lake of Genesareth.
According to the usual and entirely probable explanation they became, however, for a time disciples of John the Baptist, and were called by Christ from the circle of John’s followers, together with Peter and Andrew, to become His disciples (John 1:35-42). The first disciples returned with their new Master from the Jordan to Galilee and apparently both John and the others remained for some time with Jesus (cf. John ii, 12, 22; iv, 2, 8, 27 sqq.). Yet after the second return from Judea, John and his companions went back again to their trade of fishing until he and they were called by Christ to definitive discipleship (Matthew 4:18-22; Mark 1:16-20).
In the lists of the Apostles John has the second place (Acts 1:13), the third (Mark 3:17), and the fourth (Matthew 10:3; Luke 6:14), yet always after James with the exception of a few passages (Luke 8:51; 9:28 in the Greek text; Acts 1:13).
From James being thus placed first, the conclusion is drawn that John was the younger of the two brothers. In any case John had a prominent position in the Apostolic body.
Peter, James, and he were the only witnesses of the raising of Jairus’s daughter (Mark 5:37), of the Transfiguration (Matthew 17:1), and of the Agony in Gethsemani (Matthew 26:37).
Only he and Peter were sent into the city to make the preparation for the Last Supper (Luke 22:8).
At the Supper itself his place was next to Christ on Whose breast he leaned (John 13:23, 25).
According to the general interpretation John was also that “other disciple” who with Peter followed Christ after the arrest into the palace of the high-priest (John 18:15).
John alone remained near his beloved Master at the foot of the Cross on Calvary with the Mother of Jesus and the pious women, and took the desolate Mother into his care as the last legacy of Christ (John 19:25-27).
After the Resurrection John with Peter was the first of the disciples to hasten to the grave and he was the first to believe that Christ had truly risen (John 20:2-10).
When later Christ appeared at the Lake of Genesareth John was also the first of the seven disciples present who recognized his Master standing on the shore (John 21:7).
The Fourth Evangelist has shown us most clearly how close the relationship was in which he always stood to his Lord and Master by the title with which he is accustomed to indicate himself without giving his name: “the disciple whom Jesus loved”.
After Christ’s Ascension and the Descent of the Holy Spirit, John took, together with Peter, a prominent part in the founding and guidance of the Church. We see him in the company of Peter at the healing of the lame man in the Temple (Acts 3:1 sqq.).
With Peter he is also thrown into prison (Acts 4:3).
Again, we find him with the prince of the Apostles visiting the newly converted in Samaria (Acts 8:14)."
(Continued)