The Gospel is, in the first place, news. And not just any news; it's really, really, really good news.
It's not the Bible. The Bible contains it.
It's not the church. The church is the community of people who believe it.
It's not belief, or baptism, or repentance, or remembering Jesus, or faith, or a prayer. Those things are potential responses to it.
It's not salvation, or the best possible life, or hope, or joy, or peace, or confidence, or purpose, or security. Those are some results of it in those who believe it.
It is news. It is very, very, very, very good news.
So good, that God left heaven and packaged Himself up in flesh to deliver it through Jesus. And then he packaged Himself up in everyone who believed Jesus to deliver it some more through the Holy Spirit. It's so good, that most religious folks just couldn't buy it, and wondered where Jesus got off sharing such an impossible thing. It's so good, that even immortal and powerful angels who live in God's very presence can't take their eyes off of it.
It's far too good to contain in one sentence, and yet there are many sentences that accurately describe it. Here is but one: You are forgiven, and you have been freed from sin and death, and you can now live fully and for eternity with God.