What was Jesus’ gospel in Mark 1:14?

[TonyChanYT]

Jesus taught Nicodemus in John 3:

3 Jesus replied, “Very truly I tell you, no one can see the kingdom of God unless they are born again.” ...
14Just as Moses lifted up the snake in the wilderness, so the Son of Man must be lifted up, 15that everyone who believes may have eternal life in him.”
16 For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.

That's the heart of the gospel.

22 After this, Jesus and his disciples went out into the Judean countryside, where he spent some time with them, and baptized. ... 24(This was before John was put in prison.)

Mark skipped all these. Mark 1:

14 Now after John was arrested, Jesus came into Galilee, proclaiming the gospel of God, 15 and saying, “The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel.”

I.e., the gospel of being born into the kingdom of God.