Absolutely justifiable, beyond question. Prevention of a forcible felony that may cause great bodily harm. The disparity of force was also enough. How much you want to bet that a couple of them would have run at the sight of a gun? Then the next one to make a forward motion towards me would have gotten some extra ventilation, a shot to the pelvis gets the point across and is instantly incapacitating. Who knows, maybe his screams of agony would have motivated the others to leave.
And as far as it being illegal to carry a gun where he was, oh well. A little jail time is a little price to pay for the health of my family (this is one of the reasons I am such an advocate of OC, then you would have had 8 guys on the ground screaming like little girls). You also have to look beyond the immediate physical health of a rape victim. They will be forever changed or at worst mentally destroyed depending on the person. Alot never recover.