ISP,
Paul’s writings were directed mostly to Gentiles who had never been exposed to the Law Covenant, not having been commanded to keep it. It would not have been of much value to these people to dwell on the Sermon on the Mount as Jesus was here explaining the essence of the Law Covenant. The information found in chapters 5 and 6 of Matthew was for the benefit of the Israelite listener as they were under this covenant and had not kept it because they probably did not truly understand it. Jesus was showing them what Jehovah expected of them. Jesus said of the Jewish leaders; “Woe to you, Scribes and Pharisees, hypocrites! Because you give the tenth of the mint and the dill and the cumin, but you have disregarded the weightier matters of the Law, namely, justice and mercy and faithfulness. These things it was binding to do, yet not to disregard the other things.” (Matthew 23:23) Jesus then had to do what these men failed to do. So, yes, it was gospel! It was gospel to Israel provided they heeded his words.
Matthew 13:34, 35 states: “All these things Jesus spoke to the crowds by illustrations. Indeed, without an illustration he would not speak to them; that there might be fulfilled what was spoken through the prophet who said: “I will open my mouth with illustrations, I will publish things hidden since the founding.”” If Paul did not write any explanation of these parables, it would be because this information was not applicable to first century Christians. These things would not be fulfilled upon these Christians. Even Paul recognized that there were things that he did not understand. Note his words at 1 Corinthians 13:9. “For we have partial knowledge and we prophesy partially; but when that which is complete arrives, that which is partial will be done away with.” Complete fulfillment of prophesy would not occur in the first century. It was reserved for a later period of time.