He was the person that spoiled the assassinations attempt. Father Oswald Tesimond, approached father Henry for advice because, Robert Catesby (a lead conspirator) had told father Oswald about the Gunpowder Plot in confession. He wanted to worn England about the upcoming event but felt he couldn't because he found out about it through confession,
so he wrote to the roman church to let England know. The English authorities however, thought the father Henry Garnet and his fellow jesuit order members were the leading members in the conspiracy
Why was he called the great equavaior? Well first to know why he was called that we need to know what that even means. “Equivocation was a Jesuit logic that allowed Catholics – who may have needed to lie under oath in order to preserve their lives or those of other Catholics – to avoid incriminating themselves or others, without lying in the eyes of God” The prosecution was repeatedly complaining about the act and support of equivocation. The English authorities were insanely distrustful of equivocation, and saw it as a sinful attack of language and Jesus.