"Gross incompetence" refers to a very serious level of incompetence or lack of skills, knowledge, or ability to perform a task or job. It suggests that someone is extremely bad at what they are supposed to do. Full definition
An entire village is denied the opportunity to vote because a clerical error stemming from gross incompetence in the digital age. (nystateofpolitics.com)
If the manager of the local convenience store at the end of the record kept his files in such a state he would be fired for gross incompetence. (judithcurry.com)
We could ignore this similarity but to do so would be gross incompetence on our part as reviewers. (thedigitalfix.com)