Hi @LauraTywater ...
When an assignment is added to your course, a column is automatically added to your Gradebook. There is no way that I know of to hide and/or remove a column in the Gradebook yet keep it as an assignment on your "Assignments" index page. If the McGraw Hill assignment is worth 0 points, you could also mark the option "Do not count this assignment towards the final grade". Students have an indicator in their own Gradebook that the assignment would not be counted towards the final grade.
I know this isn't the answer you were hoping for, but I hope it helps in some way to answer your question.