As per Gaspipe Casso, Gravano was one his biggest customers in his drug trafficking business.
Is this substantiated or speculation?
It's speculation. That's why Little Man prefaces every post he makes with "as per Gaspipe Casso."

You're a good guy, Little Man. You're obviously very bright and well read. But you really have to start recognizing the Philip Carlo book for what it was: A fanboy whitewash by a hack writer.