The Gospel is the good news about what God has done for us through the life, the death, and the resurrection of Jesus Christ. Unfortunately, the good news doesn't make much sense without first learning the bad news.
The bad news is that every person in the world has a deadly disease. This disease is worse than cancer. Though cancer can destroy our body and end our life on Earth, the disease we have is more like a spiritual cancer. It can destroy our body and our soul for all eternity.
The Bible calls this spiritual cancer "sin", and it also says that each of us have it, "For all have sinned and fall short of the glory of God." The Bible also states that our sin will bring about death for each of us, as it is written, "the wages of sin is death." This includes physical death (separation from our body), spiritual death (separation from God), and eternal death (separation from God forever). This is obviously the bad news!
But the good news is that God has provided a solution for us in the person of Jesus Christ. This cure doesn't come in the form of pills or injections, it comes through faith in God's gracious work of Jesus dying on the cross. For anyone who believes, His death serves as a substitution for the horrible punishment that you and I deserve (because of our selfish disease of sin).
This was all motivated by God's love for us and His desire to have us spend eternity with Him in heaven. The alternative is perishing for the consequences of our sin for all eternity, separated from God and all that is good, in hell.
In John 3:16, this story is summarized, simply yet powerfully: "For God so loved the world, he gave his one and only Son, that whoever believes in him, shall not perish but have eternal life."