FWIW, there are several things that happen with any error. You say it doesn't change the gospel; I'm not sure that is so, because any error changes everything.
Some say that a person's sin is between that person and God, and nobody else. But the Bible talks about the Body as a whole of many members. If one member is not in unity with God, the rest suffer in one way or another as a result. The understanding of the Gospel works the same way. Every word articulated introduces truth or falsehood. God can get the basics across, and an adulterated Gospel may still contain the essence, but not the fullness.