Christians are the ones who use the Bible in order to know the teachings of God which they must obey. It is the tool which God gave to all mankind as an evidence that God surely exist and that all of His words are absolute. In the Bible, there were even records about the history which had happened such as the achievements of King Cyrus and Alexander the Great. Actually, before they were born, the Bible already recorded about them and all of their achievements.
Also, there are records in the Bible about the Earth being suspended in space as well as the water cycle. Such records were already made about 3,500 years ago which is a long time ago before scientists such as Isaac Newton who discovered the Law of Universal Gravitation in the 17th century and Perrault and Marriott who discovered about the water cycle just in the 16th and 17th century. Through this, it can prove that the Bible is the absolute words of God, not just written by men.
Regarding the gospel, this was taught by Jesus Christ whom Christians believe. Then, what is the gospel? Everything which Jesus has taught to His disciples 2,000 years ago is the gospel. If there is gospel, there is also false gospel. And false gospel came after the death of Jesus and His disciples. Everything that are being practiced after His death and are not found in the Bible belongs to the false gospel. Studying the Bible diligently will let you distinguish the truth from falsehood.