Definitions are not correct or incorrect, they are simply useful or useless, where usefulness is judged by the clarity of one’s attempts at communication.
The Gospels of Matthew, Mark, Luke, and John are accounts of the ministry of Jesus Christ, emphasizing His teachings, parables, miracles, Atonement, death, and Resurrection. In contrast, the Book of Mormon explains WHY the life and teachings of Jesus are so vitally important to the world. In other words, the Gospel writers tell WHAT Jesus did in the Old World. The Book of Mormon writers tell WHY it was so important that He came to earth and performed the Atonement to redeem us all.