In his own reddit AskMeEverything page,
https://www.reddit.com/r/IAmA/comments/80ow6w/im_bill_gates_cochair_of_the_bill_melinda_gates/He answers about cryptocurrency stating
The main feature of crypto currencies is their anonymity. I don't think this is a good thing. The Governments ability to find money laundering and tax evasion and terrorist funding is a good thing. Right now crypto currencies are used for buying fentanyl and other drugs so it is a rare technology that has caused deaths in a fairly direct way. I think the speculative wave around ICOs and crypto currencies is super risky for those who go long.
He isn't wrong in his sense, if the cryptocurrency is to have a future, they should be able to create an environment of trust between themselves and the authorities.
I think Bill Gates hasn't have enough time to study cryptocurrency, I guess he just know and read something about bitcoin that is used in illegal purposes in the dark web, but we all know what crypto really is at the moment, its benefits and advantage to fiat is explicitly stated by coins.