Just as a clarification; there is no legal definition of gross misconduct. Because of that, employers are encouraged to be very careful in what they term gross misconduct. In case of a question, the DOL is going to most likely rule in favor of the employee. Therefore, the rule of thumb I have been taught to use, and which I have indicated here before, is that if the employer (or someone else) can prosecute, then it's probably safe to consider it gross misconduct.
However, the law does not say that it has to be criminal before it is gross misconduct. That is just a line that has been found to be relatively safe in drawing.
To all intents and purposes, though, ecmst12 is correct.