Literally the Greek word for “gospel” means “good news”. In the ancient world in Christ’s time, it had royal connotations. But when the Greek Septuagint was written (probably started to be written circa 300 BC), the word was used with its default meaning. Remember, the New Testament quotes the Septuagint, so we can’t just dismiss it.
We find the word Gospel in several verses:
2Sa 4:10 when someone told me, saying, ‘Look, Saul is dead,’ thinking to have brought good news, I arrested him and had him executed in Ziklag—the one who thought I would give him a reward for his news.
2Sa 18:20 And Joab said to him, “You shall not take the news this day, for you shall take the news another day. But today you shall take no news, because the king’s son is dead.”
2Sa 18:22 And Ahimaaz the son of Zadok said again to Joab, “But whatever happens, please let me also run after the Cushite.” So Joab said, “Why will you run, my son, since you have no news ready?”
2Sa 18:25 Then the watchman cried out and told the king. And the king said, “If he is alone, there is news in his mouth.” And he came rapidly and drew near.
2Sa 18:27 So the watchman said, “I think the running of the first is like the running of Ahimaaz the son of Zadok.” And the king said, “He is a good man, and comes with good news.”
2Ki 7:9 Then they said to one another, “We are not doing right. This day is a day of good news, and we remain silent. If we wait until morning light, some punishment will come upon us. Now therefore, come, let us go and tell the king’s household.”
When Christians insist heavily to make “gospel” a proper noun with one defined meaning, the burden of proof is on them to show it is scriptural.