"a voice of one calling in the desert, prepare the way for the Lord, make straight paths for him." (Mark 1:3)When Mark identified John the Baptist with Isaiah's "voice calling in the desert", Mark names John as the herald announcing the coming of the LORD (see Isaiah 40). The word "Lord" in our language or in ancient Greek may be ambiguous, but the word "LORD" in the original Hebrew was not ambiguous at all. It was the holy name of God, not to be casually pronounced, a name reserved for God. When Mark identified "the voice calling in the desert" as John the Baptist, Mark thereby implies that the one John announces is the long-awaited LORD from Isaiah's prophecy, the God whose arrival was good news, "good tidings", or (in older English) gospel.
There are accounts of Jesus' life that introduce Jesus at his birth. Mark's introduction starts with his identity. Over the centuries we may have lost sight of the boldness of that introduction, but a first-century Jewish audience would probably get his point: the most joyful event in the world was unfolding, one which could not leave the world unchanged. God's arrival in our world is the good news, and the kingdom of God is among us.