I believe that the purest answer lies within what the word "euangelion" meant to the original, First Century hearers. It was a widely used term which had no religious connotations at the time, and was understood as the heralding of one of three things: The Birth of a King, The Triumph of a King over his enemies, and a Regime Change (New King/New Order).
Hence, when the early church used the word "Euangelion"/Gospel, it was understood that a new King had been born, that He had defeated His enemies, and that there was indeed a regime change when Jesus was officially coronated through the finished work of the cross. Making those claims about Jesus was a revolutionary declaration that "Jesus is King" (and Caesar is not).
It meant so much more than today's typical "Evangelical" message. While there are many components of "The Gospel", I believe that what I have described is the crux of the Gospel. Just as "Hail to the Chief" is used to herald a U.S. President, "Euangelion" was a word reserved specifically for the ruling Caesar (or his successor) of that day.
Peace & Love!