"The gospel" in Mark 1:15 is in the context of Christ's words, "The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel." It's necessary to understand what time the Lord is talking about, in order to see what "the gospel" here means.
So, What time is He talking about?
In Daniel 9:24-27, the angel Gabriel told Daniel:
24 Seventy weeks are determined upon thy people and upon thy holy city, to finish the transgression, and to make an end of sins, and to make reconciliation for iniquity, and to bring in everlasting righteousness, and to seal up the vision and prophecy, and to anoint the most Holy. 25 Know therefore and understand, that from the going forth of the commandment to restore and to build Jerusalem unto the Messiah the Prince shall be seven weeks, and threescore and two weeks: ... 26 And after threescore and two weeks shall Messiah be cut off, but not for himself: and the people of the prince that shall come shall destroy the city and the sanctuary; ... 27 And he (that is, the Messiah) shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease ...
The link between the 70 weeks of Daniel and the time Jesus spoke of is this:
Christ was crucified, "cut off," at the end of the 69th week of years. After his resurrection, he "confirm(ed) the covenant with many for one week" (of years), during which week Saul of Tarsus was saved and became the apostle Paul, Christ's apostle to us Gentiles. So, when Jesus said "the time is fulfilled," that was at the beginning of his 2-year ministry (see references to the 3 Passovers leading up to his crucifixion, John 2:13, 6:4, and 11:55) and was the end of the 5th year of the 69th week of years, according to Daniel's prophecy.
The gospel, (or, the good news Jesus preached,) as shown in Romans 1:1-7 and I Corinthians 1:17-2:16, for example, was Christ crucified, whom God raised from the dead the third day: Christ being the Son of David, as Mary his mother was of the house and lineage of David (Luke 3:23-32), and being the Son of God, as the Lord Jesus was conceived in her womb by the Holy Spirit (Matthew 1:18).
That therefore is the gospel of the kingdom of God, the gospel that Jesus preached, the gospel which he called us to believe, and the gospel he commanded us to obey, as it is written, "Repent ye, and believe the gospel."
Why does the call to repent and believe the gospel of the kingdom of God continue to go forth? Seeing that the Lord Jesus' 2-year ministry was about showing infallible proofs that he was the Messiah, according to the prophecies of the Hebrew Scriptures that preached of him (Luke 24:44-48); and that his 40-day ministry after his resurrection was likewise showing many infallible proofs that he, the Messiah, the Anointed, the Christ, had fulfilled all that the prophets and the law of Moses prophesied of him before he ascended to the right hand of God our Father in heaven, and began his reign in his everlasting kingdom (Luke 24:49-53, Acts 1:1-3, Revelation 1:5, Psalm 2); and that, one day, Christ shall return in the clouds of heaven, the same way he ascended up into heaven, to judge the living and the dead (Acts 1:9-11, 17:22-31); the call continues to go forth, to repent of our sins, to turn from idols to serve the living and true God, and to wait for his Son from heaven, whom he raised from the dead, even Jesus, who delivered us from the wrath to come. (I Thessalonians 1:9-10)