Since the consequences to the rest of scripture are indeed dire here, where does this rule come from? A Gospel-only approach nullifies much of the full revelation by God we are given in scriptures.
"Gospel-only" is a distortion of what i said, FK. Gospel-first would be more like it. The rest of the Bible are visions and audible experiences of mortals receiving words from God. The Gospels are not dreams or trans-related visions...but real living word of God walking on earth. There is a qualitative difference. Nothing comes close to the Gospels. They are the filter through which everything else in the Bible is judged and brought into harmony.
Again, the Jews did not recieve full revelation until Christ appeared. The OT was a gradual revelation. It doesn't mean it wasn't true; it just wasn't full. The revelation of the OT leads and foreshados Christ with various "types." the presence of Logos in the Ot becomes apparent only after the Gospels were written.