“It seems to me patently absurd to suppose that the whole of scripture can be adequately reduced to the dimensions of a single concept such as ‘gospel’.”

I would like to hear (or read) you define ‘gospel’ before I offer a comment on this statement. If that is practical in terms of brevity.

“That is why in popular books and sermons scripture is routinely filtered, screened, distorted, and allegorized in the interests of preserving a marketable ‘gospel’.”

Would you mind listing some of these books you are alluding to so I can have a better understanding of what you mean by filtered, screened, distorted, etc.?

“But the main point I would make in response to your argument is that ‘gospel’ in scripture is not a univocal, universal religious principle. The ‘good news’ is always a contextualized announcement about something God has done, is doing, or is about to do.”

The ‘good news’ maybe contextualized according to its setting, but I would argue that it is not univocal. The good news is always the same thing in my mind from Genesis 1 - “So God created man in his own image…”; Exodus 19 - “…you shall be my treasured possession among all peoples…”; Psalm 53 - “God looks down from heaven on the children of man to see if there are any who understand, who seek after God”; Isaiah 44 - “I have blotted out your transgressions like a cloud and your sins like mist; return to me, for I have redeemed you”; Jonah 4:11 -“And should not I pity Nineveh, that great city, in which there are more than 120,000 persons who do not know their right hand from their left…”; Matthew 11:28 - “Come to me, all who labor and are heavy laden, and I will give you rest”; Romans 8:14-15 - “For all who are led by the Spirit of God are sons of God. For you did not recieve the spirit of slavery to fall back into fear, but you have recieved the Spirit of adoption as sons, by whom we cry, ‘Abba, Father!’” No matter the context or the audience the ‘good news’ is always in my mind the same call, a call to return to God. A call to be in right relationship with Him. A call to have Him as you God, as we are His people. For me this is done, once and for all (Hebrews 1:1-3; 10:10), but this discussion is for another time and place.