That's not at all what they said. They said "Clinton and her colleagues...were extremely careless." Gross negligence is a term of art with a very particular legal meaning, even if there's some vagaries in the case law. The two aren't the same. Gross negligence can include extreme carelessness, but it's generally accepted to go well beyond just that.

They certainly did not say that there was 'gross negligence'. That would have meant a case.