When I was in Bible College I often heard an old preacher tell the students, "The main thing is to keep the main thing the main thing." Of course he wasn't the first to say it, but I loved it. I still do. Recently someone I respect asked me, "is it possible that within the 'gospel-centered movement' some people are making the main thing the only thing?"
It is a great question, and I think it does point to a problem of unhealthy reductionism among some well-meaning brothers and sisters. I believe this brother was essentially saying, "Look, our people need to know what their hope is before God. This is of first importance. But, they also need to know how to pray, fast, love, give, fight, and serve." Of course, I agree with this sentiment.
There is more in God's word than the gospel. God has given us his law to show us the way, uncover our corruption and condemnation, and point us to our need of redemption. There are commands to be obeyed, there is wisdom to learn and practice, and affections to feel and be moved by. But, the law itself is unable to create within us new hearts, or empower us to obey its demands. So let me say it this way: The gospel is the main thing, it is not the only thing. However, it is the only thing that brings life, power, and transformation. The gospel isn't everything, but it does connect to everything, and preachers and teachers in the church must be able to show that connection lest we allow the church to drift (or even be lead) into various kinds of hopeless, powerless legalism.
Let's take one example. I need to learn how to pray, but I also need to learn, and be reminded, that I can only pray because Jesus has made peace between God and sinners like me. I need a practical method for praying, but I also need the assurance that when I fail to pray, God's love for me is secure and not based upon my performance. I need counsel on how to pray without ceasing, but I need the confidence that Jesus prayed perfectly in my place, prayed on my behalf, and currently intercedes for me. These gospel principles don't merely complement the command to pray, that satisfy it. They do not remove the need to pray, they give freedom and power to approach God with boldness. Without these gospel principles we are left to our own devices, and at least implicitly encouraged to trust in our work more than God's grace.
The best teaching of the church preaches the "whole counsel of God," unpacking all of the subject matter available within, but does so with the aim of grounding the hearers in the gospel. When we fail to do this we show that the functional main thing is the act of teaching or learning, rather than the gospel itself.