You're right: The Gospel is true -- and since it is, it doesn't make sense for people to turn their backs on Jesus. Jesus said, "If you hold to my teaching.... you will know the truth, and the truth will set you free" (John 8:31-32).
But there are at least two major barriers that keep people from giving their lives to Christ, even when they believe the Gospel might be true. The first is the barrier of self-will -- that is, their determination to control their life instead of letting Christ control it. They realize that if they turn their life over to Jesus, their goal will be to do what God wants them to do -- and they don't want that.
The second major barrier is harder for us to see -- but it's even more powerful. This is Satan's hold on the world, and his commitment to do everything he can to keep us from Christ. One way he does this is by blinding us to the truth, and convincing us it isn't worth it to follow Jesus. The Bible says that Satan "has blinded the minds of unbelievers, so that they cannot see the light of the gospel" (2 Corinthians 4:4).
Does this mean it's hopeless to witness to your unbelieving friends? No, not at all -- because God can break through those barriers. Pray for them, and ask God to use you to point them to Jesus Christ, who alone is the hope of the world.