Unquestionable truth (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unquestionable truth</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for gospel that I've seen before include "sacred text" , "The teachings of Christ in a Book of the New Testament" , "The teaching of Christ" , "Christian doctrine" , "Evangel" .)