This is a question that I’ve been trying to answer for a long time now. It seems there are a lot of opinions about what the word meant in the first century, what it refers to in the Bible, and what it means today. Tim Keller just recently wrote a great article about the unity and diversity in Scripture regarding “the gospel,” and it’s definitely worth checking out:
http://www.christianitytoday.com/le/2008/002/9.74.html

But, I want to address something that I haven’t seen much good explanation for. Galatians is blatantly about “the gospel,” so much so that to reject the one true gospel is to deserve damnation. Obviously, Paul thinks “getting the gospel” is pretty important. But, there is an interesting twist to our typical understanding of what the gospel is in Galatians 3:8 - “And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham…” So, whatever the gospel is, it was preached to Abraham. If our gospel is simply “Jesus died and rose again” that doesn’t really make sense of the passage. Paul goes in the next verse to explain what he means by the gospel that was preached to Abraham: “In you shall all the nations be blessed.” The promise was given to Abraham, that all nations on the earth would be blessed through him. That promise was and is good news. “The gospel” contains that promise.

Obviously, the gospel is part of a much bigger story than the story of Jesus. We have to understand the promise to Abraham to understand Jesus. Israel was given the responsibility (mission) to be different among and a light to the nations, and therefore to point everyone to Yahweh. But, Israel failed. Jesus perfectly fulfilled Israel’s original mission.

Each story points to Jesus, but each story is unique, on its own, within the larger story. Ultimately, all nations are being blessed through the seed of Abraham (Jesus). And, we have been entrusted with the gospel to participate in God’s blessing of all nations.