Currently the gospel is refused because it is already covered by gospels. But the two, while very related, are not the same. gospels covers the four accounts of the life of Christ. gospel refers to the message of Christianity, not the books that talk about it.
Mark 16:15 says to "go into all the world and preach the gospel"
Whether we're debating about what the gospel is, talking about Person X's understanding of it, or have a question related to it, we need a tag for it!
So please, let's allow gospel!