This is what I see happening. With GBTC coming to closer to parity with NAV just as you said and with the upcoming decision from the Winklevoss ETF (COIN) and the SolidX Bitcoin Trust ETF (XBTC). Let's say they say yes, then you should see an influx of Bitcoin being purchased which should drive up the price of Bitcoin, which would in theory then drive up the price of GBTC. If they come back and say no, then in theory GBTC would once again go up in price and be further from NAV as there would not be a COIN or XBTC ETF available to participate in. Either way, unless BTC falls dramatically for some reason GBTC should only go up from here with both good or bad news from the upcoming SEC announcements.
I think the GBTC might end up significantly lower though. A rising bitcoin tide will raise all boats. but would it raise GBTC fast enough to offset that? I thought there were some big advantages for almost all investors in choosing an ETF over GBTC. Is there any reason someone would prefer GBTC? If not then the only reason anyone would buy it would be because it's cheap enough that going to the trouble of getting the actual BTC out would be worth it.