The word "gospel" occurs in the New Testament over 90 times. Jesus said people were to "repent and believe in the gospel.”
What does the word "gospel" mean? In its simplest form, gospel means "good news." It is connected with ancient Greek meaning someone who brought good news. For example, a messenger might bring good tidings of a victory or other great news.
The gospel truly is good news! The gospel of Jesus Christ is the Good News of Jesus Christ -- that Jesus Christ came into this world to save us who are sinners. God sent His own Son to redeem the world. We don't have to be perfect -- we don't have to die ourselves -- we just need to believe in Him who the Father has sent.
Even in the titles of the books of the Bible, it talks about "The Gospel According to Mark." Another way of looking at that is to say "The Good News According to Mark."
So you can ask your friends -- Have you believed in the good news? Have you believed the gospel?