I don't know if this is original with me, but if it isn't I can't remember where I read or heard it.
Is the Gospel simple? It can be. So simple a child can understand it. But simplistic? Not really. I can whistle the first few notes of Beethoven's 5th symphony. (I can even type them -- da da da DAA). If you listen to the symphony you hear these notes repeated often, but there is a whole lot more going on.
So the Gospel of Jesus Christ is like a symphonic theme, so simple a child can understand and repeat it, but capable of an immense amount of variation and exposition. A large percentage of the uncountable sum of God's thoughts have to do with it.