The other day I wrote a post "What is the gospel?"
Today I read a post that has a very helpful answer. It summarizes a series of posts about the gospel as taught by Jesus, in the books we call the "gospels" - Matthew, Mark, Luke, and John.
You can read this excellent summary, Gospel in the Gospels, at Jon's Journey. Then, of course, you'll want to go back to the previous posts in the series to read all the details.