They seemed to think you would be able to buy merchandise at his shows with Gacktcoin. Whether he explicitly said that or just let them think it, I don't know. But when he went around telling the world that this was the future, of course his fans were the ones who'd listen most.
At any rate, the part that's illegal or near-illegal was dumping all his holdings and cashing out right before the coin lost 99+% of its value. I believe it was millions of dollars' worth of looting all told - though that presumably includes all the ringleaders, not just Gackt.
So there's a good reason to not like him, instead of the bullshit kids on twitter keep going on about ¯\_(ツ)_/¯