"Gross misconduct" strikes me as a catch-all phrase when they can't get you for anything else. Obviously walking off with the day's takings or pouring coffee over the MD's head whilst telling him what an utter ****** he was would be classified as such by any reasonable person, but some firms treat the most trivial thing as "gross misconduct".
This reminds me of my time in the RAF when, if they couldn't get you for anything else they would use section 69 of the Air Force Act 1955, "...conduct prejudicial to good order and Air Force discipline". What constituted such conduct was at the whim of the officer hearing the charge. If any airman was unlucky enough to lose his wallet containing his ID card he was automatically charged with, "...losing by neglect RAF Form 1250, conduct prejudicial to good order and Air Force discipline". If I remember rightly the standard penalty was a fine of 10/-, which covered the cost of the replacement.