Gross misconduct is misconduct so serious that the employer is entitled to dismiss for a first offence. What that means depends in part on the individual workplace. Some offences will almost always be gross misconduct, whatever the character of the workplace. For instance, fighting, stealing, arson and deliberate falsification of time-sheets will mostly be in that category. (Though even in these extreme cases there is a need for some commonsense. If it is standard practice in a particular workplace that you always claim an hour’s overtime if you have stayed past your finishing time at your supervisor’s request, then a time-sheet that is on its face inaccurate may not in fact be dishonest.)
Other conduct may be regarded as intolerable in one workplace, but widely accepted in another. Attitudes to smoking make a good example. Some employers will slap your wrist for an infringement of the rules about where you may smoke, but if you work on an oil rig you can expect to be on the next helicopter home for smoking in a prohibited area. Drugs and alcohol provide another example: if you are an airline pilot, attending for work under the influence will almost certainly be a sacking offence. For other employers, alcohol in particular may not be taken seriously unless it starts to affect performance. (And if part of your job is to take clients out to long boozy lunches, not drinking might even be regarded as problematic.)
In general, employers that want to enforce higher than average standards have to make it clear where the limits lie. The stricter they want to be, the more careful they must be to make sure everyone is aware of the rules. If your company handbook lists fighting, stealing and gross insubordination as gross misconduct, then you can expect some warnings before you are sacked for lateness. But if the handbook spells out that in this industry punctuality is essential and lateness will be treated as gross misconduct, then gross misconduct it almost certainly is.