These critics say that the gospel is not the only part of the Bible that is to be preached and obeyed. If you focus on the gospel too much, this may lead to disobedience in other "non-gospel" parts of scripture.
The Christian life is not so much a life of believing the gospel, but obeying the Bible. (BTW, if you disobey the Bible, you are not believing the gospel in some way.)
Here's an example of this thinking:
"The Christian life is described as a life of faith in the power of the Spirit that faith rests in the Gospel for salvation and ultimately in God Himself. That faith is increased as we study and hear the Word, not just the Gospel, but the whole counsel of God. To take a part (the Gospel) and to make it the whole is a logical error. The Gospel may be the most important part of God's Word, but it is not the whole. It may be the entry point and must constantly be appreciated (as in communion, for example), but we do need the entire counsel of God, much of which is only indirectly connected to the Gospel message itself."
I agree with much of what is said here. Don't get me wrong. We need to have faith in the Spirit's power and grow in our faith as we read the Bible as a whole unit, not just sections of it. Yet, there is one dominant theological presuppositions that I think is informing this idea:
Dispensationalism - The dispensational mind divides everything up into neat little sections and a whole worldview is formed through this lens. See how the gospel is the entry point and afterwards, the only real value that it has is appreciation through communion. Thankfulness for a past event that really doesn't have much value on the here and now or the future. Dispensationalists, I believe, are partly guilty for the mentality that Christians have formed over the years in dividing their Christian life from their private life; their sacred tasks from their secular tasks; evangelism from discipleship, and the list could go on. This is clearly the mentality in the above quote. Gospel is for getting in, then there's the rest of the Bible that must be learned and obeyed.
A holistic approach is better - The above statement rightly says that God is the ultimate object of our faith. His story and plan is the gospel and therefore, His gospel is the way in, it is the way, and it is the destination - all accomplished and consummated in Christ! The whole Bible is about this plan and every story, prophecy, letter, Psalm and Proverb is connected to it in some way. I heard a guy try to say that if he was preaching on "the tongue" as mentioned in:
James 3:6 And the tongue is a fire, a world of iniquity: so is the tongue among our members, that it defileth the whole body, and setteth on fire the course of nature; and it is set on fire of hell.
That would not be preaching the gospel. In that case, all you would be preaching without the gospel, is moralism and legalism. "Don't sin with your tongue! Don't gossip! Don't curse! That's bad because the Bible says so!"
So what? The Bible says so, isn't a good enough answer. Why does it say so? What is the cure? Christ is the only cure and the gospel is the only way to apply the cure! Every part of Scripture is affected by the gospel. So, if you were to talk about the gospel too much, how would that eclipse James' teaching on the tongue? If Christ is the way the truth and life, (which is part of the gospel), then that should translate into "walk in the way of Jesus who did not revile when he was reviled; speak the truth because He is the truth that you have believed; and speak in a way that gives life and edifies if you have received this life".
When you look at the Bible from a Dispensational point of view, then you're naturally going to look at the gospel as one phase or one section or one part of the whole thing instead of seeing the gospel as the point of the entire Bible. So, if someone were to talk too much about eschatology, I could see how that would eclipse the rest of the Bible, because you're taking the end of the gospel story and making it the most important part. The whole counsel of God is the whole gospel story from the past, the present and the future. If there's something in the Bible that is completely disconnected from the gospel, it probably doesn't belong in the Bible. But because we know the whole Bible is from God, it's all about His gospel story.