One gigantic correlation between the Gospel of John and Revelation is John's (Disciple John's) record of Jesus's encounter with John the Baptist.
John 1:29, 35-36
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on the morrow John seeth Jesus coming unto him, and saith, 'Lo, the Lamb of God, who is taking away the sin of the world;
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On the morrow, again, John was standing, and two of his disciples,
and having looked on Jesus walking, he saith, 'Lo, the Lamb of God;'
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(Disciple) John thought this was important enough to record while all the other disciples left it out, and this imagery permeates Revelation.
Revelation 5:6, 8
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and I saw, and lo, in the midst of the throne, and of the four living creatures, and in the midst of the elders, a Lamb hath stood as it had been slain, having seven horns and seven eyes, which are the Seven Spirits of God, which are sent to all the earth,
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And when he took the scroll, the four living creatures and the twenty-four elders fell before the Lamb, having each one harps and golden vials full of perfumes, which are the prayers of the saints,
Revelation 7:14
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and I have said to him, 'Sir, thou hast known;' and he said to me, 'These are those who are coming out of the great tribulation, and they did wash their robes, and they made their robes white in the blood of the Lamb;
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I just picked three examples but there are many more references to the Lamb in Revelation.