There are links with the gospel, but also developments The purpose of John's gospel was evangelistic, the purpose of 1 John is assurance.
The message 1 John summarized:
• Belief
• Behaviour
• Confidence

Confidence
In approaching God
That he hears us
That he will answer

Behaviour
The sin that leads to death - the sin of the non-Christian - I.e. that our lives don't show that we belong to him.

Belief
We know him who is true - the Son of God has given us understanding. Jesus is the true God.