Wow, I figured it always traded at a premium. 18% discount is huge, am I missing some downside of this arb opportunity (buy GBTC sell short BTC or BTC futures)?
GBTC has no redemption mechanism, you can wrap more BTC into GBTC but not the other way around. For ETFs this is what solves the arb.
You can do your arb but you still need to offload GBTC at some point and the only option is to another buyer of GBTC.