Grievous bodily harm means serious bodily injury. It does not mean minor injuries such as a black eye or a bloody nose, but does mean fractured or dislocated bones, deep cuts, torn members of the body, serious damage to internal organs, or other serious bodily injuries.
If you reasonably believed that Death or grievous bodily harm was about to be inflicted upon yourself, then you were permitted to use any degree of force believed necessary to protect against death or grievous bodily harm. The fact that you used excessive force or that used a different type of force than that used by the attacker does not matter.