Not to detract from the main point of the question (what the gospel means in this verse), I feel a need to start with the point about "believe" (this gospel). If there is clarity at the outset as to the role of belief, then the individual may gain vital insight into the actual gospel message. Whereas if it is the other way around - knowledge of what the message is, followed by believing it - something crucial could be missed.
Take the epitome of believing faith - Abraham. God told him to do something that could have undermined his belief (faith) in God's promise that through the miracle child of promise, Abraham would become father to many nations. Yet such was Abraham's trust in God's promise, he reckoned God would resurrect the lad back to life if he kept faith with God's command, doing it - Hebrews 11:17-19. Abraham was not told this. In faith, he reckoned that God's command would not negate his promise, so without knowing exactly how God would deal with a sacrificed child of promise, he took the step of obedient faith. He did not know God was testing his faith until he was told to stop, and to use the ram caught in the thicket instead.
Take the way God's people were to be prepared to receive the Messiah in faith; John the Baptist told them to repent, and to be water baptised, in readiness for the promised One. They did not know what the Messiah's gospel message to them would be, for his ministry had not yet started. All they knew was that the messenger had come before Messiah, to prepare his way, and they were to repent and be baptised. Those who took that step of obedient faith were then prepared to receive the Messiah, and they did.
Time and again in the Bible, believing faith has to be exercised, then understanding comes. Clarity begins to shine through the confusing mists when there's a repentant heart and a desire to be obedient, irrespective of not understanding what's really going on.
Now consider how human nature resists this way of God - the way of the gospel. People want to have knowledge of what's entailed, what the details are of the plan, and then they will decide whether they are prepared to go along with it.
This means that the answer to your question is that when the person is prepared (by a humble, obedient desire to please God), then they will take the step of faith first, and understanding of what, exactly, the gospel is, will break through. All they have to know is that Jesus, in his person, IS the good news and he is to be followed, no matter what. All they have to know is that Jesus is the Son of God (in the way Peter did in Matthew 16:13-17) and believe that, then the magnitude of the gospel will break into their lives.
Yes, there is a basic minimum of what is to be believed, as stated in 1 Corinthians 15:1-4, and that minimum is based on believing Jesus to be the Son of God who died and was raised back to life, that sinners trusting in what he did will be pardoned and live eternally, in Christ. However, just knowing the facts will never result in salvation. After all, the demons believe there is one God - and shudder. The demons had to obey Christ's commands when he walked on earth, knowing he could send them to eternal punishment with a word, instantly. It's not enough to know who Jesus is if you are not longing to follow him obediently. The heart has to be moved so that the step of faith (trust) in Jesus causes that repentance and following which is proof that the gospel has been believed.
The gospel in Mark 1:15 was Jesus telling those ones prepared by John the Baptist to follow him, for the kingdom of God was at hand, the time having been fulfilled. Once they believed that simple command, they would enter into the kingdom of God and discover Christ to be its king.