The Christian Gospel is the good news that Christ brought. To summarize, it's The life of Jesus Christ our Lord and Savior and His ministry.
This resource does not intend to replace the Bible, nor does it intend to add onto the bible. I suggest using one of the
mainline bibles that have been known as trustworthy (ESV, NKJV, etc).
Back in the old testimate, people had to sacrifice animals for each sin they did. Jesus fulfills this by being the ultimate sacrifice, paying for all of our sins as long as we believe in Jesus.
Jesus Christ fulfilled this by dying on the cross, going to hell, and conquering death and ressurecting 3 days later. He showed himself to Mary, the 12 apostles, and then 500 more people (as accounted in the Gospels).
If Jesus hadn't come down, we would all be condemmed to hell for eternity, or we would have to sacrifice an animal every year on the Israel temple, and if we couldn't, our sins wouldn't be forgiven.
However, since Jesus has risen from the dead, all of our sins have been forgiven in Christ.
Jesus has done a lot more in his lifetime, like starting his own ministry (now the Christian Church), many many miracles, and a lot more. I implore you to read the 4 gospels for yourself, which are linked in our sources.