Because the Gospels didn't exist when the Apostles went out to preach, how could it?
The 'Gospel' is the good news that Christ has come, that there is forgiveness from sin through repentance, and that Christ made this possible via his substitutionary sacrifice - paying the penalty for my sin. The preaching of the time was that the Kingdom of God was here and all of the scriptures of that time were being fulfilled right in front of them.