An act so serious that it justifies summary dismissal (instant firing) without notice.
Examples include theft, physical violence, or serious intoxication at work. However, the burden of proof is high. You must still conduct an investigation and disciplinary hearing before dismissing the employee.
Usually, no. Persistent lateness is generally "Misconduct" or "Poor Performance," which requires a series of warnings (Verbal, Written, Final Written) before dismissal. Gross Misconduct is reserved for single acts so serious (e.g., assault, serious fraud) that the employment relationship is irreparably broken immediately. Treating lateness as Gross Misconduct without prior warnings will likely be deemed unfair by the WRC.
You cannot literally say "You're fired, get out" without a process. Even in cases of clear Gross Misconduct (theft, fighting), you must suspend the employee on full pay, conduct an investigation, and invite them to a disciplinary meeting where they can respond to the evidence. "Summary Dismissal" means dismissal without notice period (pay), not dismissal without procedure. Firing on the spot often leads to the employee winning an Unfair Dismissal case on procedural grounds, despite being guilty of theft.