Where does it state that was his IQ? He did feign insanity..
"His brother Louis insisted that Vincent had a tested IQ of 69. His mother Yolanda, when questioned about her son's alleged leadership of the Genovese crime family she stated, "Vincenzo? He's the boss of the toilet!" A psychiatrist retained by his relatives said in an affidavit that Vincent "suffers from auditory and visual hallucinations and delusions of persecution." Gigante had two families and lived in two different places. He had allegedly been mentally troubled since the 1960s with a below normal IQ of 69 to 72."
Wikipedia (I know)