One Mormon author who I’ve long derived inspiration from is Ogden Kraut, who has written many books on various gospel principles. One which helped clarify my understanding a lot is called The Church and the Gospel. The title is pretty self-explanatory. In a nutshell, Kraut is concerned here to show that the two terms are not necessarily synonymous. A person might have legitimate concerns or issues with the church at any given time, but may yet retain faith in the unchanging Gospel principles.
So, when we speak of someone “leaving the gospel,” we have to be extra careful to clarify exactly what that person is thinking and what his aim is. Unless he is completely changing religions or leaving religion altogether (which might be understandable in some cases), don’t take it for granted that a person’s difficulties or disillusionment with the church equates to leaving the gospel.