What is being heard (in the phrase "faith comes by hearing") is not specified. If we lift it from its context, without considering the rest of what Paul says, we still get a truism: Faith, or belief, follows hearing (or reading). However, the rest of the verse says, ". . . and hearing by the word of God." This relates directly to faith. The faith, the belief, that God is interested in will come from a specific message—one that has its origin in God, not the world.
Therefore, it is the message of the Bible because it is the Word of God. It is not limited merely to the gospel—or even to the New Testament—but the whole Book is part of the gospel! A number of commentators say they believe that it is more understandable if the very last word of verse 17 is translated into the word "Christ." "Word of God" is not wrong, but they feel it is more specifically correct as "Christ" because He is God.
In the context of the book of Romans, the gospel is called the "gospel of Christ," because Paul says, for instance, in Romans 1:16, "For I am not ashamed of the gospel of Christ." In other words, it is the message that He brought.
It is His message that produces the faith that will save.