GBTC is becoming an ETF, so shortly the answer will be "nothing". But, currently it's not an ETF or equivalent because you can't redeem the shares of the ETF for the underlying asset. This has resulted in the price of GBTC diverging significantly from the price of bitcoin since you don't have that natural ability to arbitrage away and price differences with redemptions
GBTC has two problems:
1. fees are absolutely insane
2. it doesn't actually track Bitcoin price
A proper ETF which aims to be competitive might be much closer to actual BTC ownership
2 will be solved by arbitrage (the discount will evaporate). 1 can be solved by competition, assuming GBTC holders actually switch to lower fee ETFs once they see the discrepancy in fees.
It will track the price now that it is an ETF, that's solved. Fees are still insane so it will slowly dwindle as people sell it in favor of other options, but it will still exist for a long time because people can't sell their existing holdings without triggering huge capital gains taxes.