Why are you surprised that none of the 100 men who answered an advert to come along and take part in such a disgusting scenario, and then either lost their nerve or were worried about the possible consequences for them in having already been implicated with that evil monster, felt the desire to then go and tell the police and answer some very difficult questions and possible accusations? Not to mention the fact that, if it got out that they had reported it, they would have instantly made themselves some serious enemies amongst their underworld community of violent criminals.
It's nothing like when an innocent passer-by suddenly hears/notices something horrific and immediately alerts the authorities - which I fully believe that a great many members of the public of both sexes would instinctively do.
I highly doubt that many of those men who knew exactly what they'd come along for chose to refrain because they suddenly discovered that Gisele hadn't actually consented and they fully respected her absolute right to her own bodily and sexual autonomy. Rather they would have realised that the clear lack of consent could have repercussions for them, if/once they were found out. They still doubtless couldn't have cared less about Gisele herself.
Maybe they were every bit as evil as the men who went ahead with the crime, but just smarter, more cautious and more self-aware.