Some are surprised when they read verses like this in Mark which explicitly state that Jesus, while in the flesh, preached the gospel. The part that puzzles some is their limited concept of what exactly the gospel is.
Listen careful to I Corinthians 15:1-4 as Paul defines the gospel - "Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand, by which also you are saved, if you hold fast that word which I preached to you--unless you believed in vain. For I delivered to you first of all that which I also received: that Christ died for our sins according to the Scriptures, and that He was buried, and that He rose again the third day according to the Scriptures."
There is no denying that the gospel message includes--in fact, it centers upon--the death, burial, and resurrection of our Lord. However, although this is the core or foundation of the message, there is more to the gospel than this.
The word gospel simply means good news. In this broad sense, the entire New Testament is the gospel (cf. Gal. 2:11-14). Although Christ's brutal death does not appear to be good news on the surface, those who know the Scriptures understand that His death atoned for the sins of the world (I John 2:2). He suffered greatly because of His love for mankind, and He was victorious over death. What Jesus' death made possible (namely, a way for man to be saved from his sins) is certainly good news; in fact, it is the best news!
But, as Jesus began His earthly ministry, His disciples did not understand that He, as their Messiah, would be crucified, buried, and raised from the dead. When Mark wrote about Jesus preaching the gospel in 1:14,15, he certainly did not mean that the Lord was expounding in detail upon the good news of His approaching death, burial, and resurrection. No, Jesus at that point in His ministry was likely just declaring (as John had been) the good news that the kingdom was near and that they needed to repent and prepare for its coming. The time had finally come (Gal. 4:4)! God's kingdom, the church, was very close to being established (and it would be established on Pentecost following Jesus' resurrection). The people needed to believe the gospel or good news about the coming kingdom.
We today must also believe the gospel or good news regarding Jesus and His church (kingdom) that has come, and we too must share the good news with others!