We as Christians know the Gospel, for it is the proclamation of good news, given by the LORD God in the scriptures. Note that it's GOD that has given this message to us. We didn't think this message up ourselves: it's origin is GOD.We surely didn't create the Gospel but it has created us: by believing the Gospel, we have been born of heaven, rejuvenated spiritually, regenerated by the Holy Spirit Himself, based on the atonement of the LORD Jesus Christ's death on the cross, and His resurrection!
One of the best definitions of the Gospel is found in
1 CORINTHIANS 15:3-4:
For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; And that he was buried, and that he rose again the third day according to the scriptures: