Normally you should not divulge the reason for the dismissal, however if it has a bearing on the employee’s ability or suitability to perform in the new role, then you may have a duty to inform the new employer. For example, if the gross misconduct is a proven case of theft, disclosing that fact might be appropriate if the employee’s new job is in a working environment where large sums of money are handled regularly and/or where the opportunity for theft is frequent.