GBTC is a double play - you're not just betting on the price of Bitcoin, but also the premium to net asset value (NAV). So you really want to try to line up both where you buy Bitcoin at a low AND GBTC at a low premium to NAV and hope to capture both a rise in Bitcoin AND the premium. It's an odd instrument that I wouldn't recommend to most people, but if you know what you're doing it can be a nice play.