GLD probably hasn't done gold any favours though. I wonder whether a Bitcoin ETF will do the same.
Well, one big difference, and what would be absolutely critical for Bitcoin ETFs and Trusts moving forward, is 100% on-demand transparent auditing of holdings. That has to be a thing, and it should be trivial and built-in. If Bitcoin exchanges and brokers can expose it easily (well some have chosen to anyway), then COIN or any of the other ETFs should be able to make that happen on any given Sunday.
Otherwise it'll just become like Gold ETFs long term.... no transparency, no proof of holdings, fractional reserve trading, naked short-selling, etc.
The COIN ETF has an ongoing financial auditor and will publish "proof of control" of the cold storage coins by a message signed with the private keys, on a monthly basis. This proof of control will be published on their website. Their cold storage system was designed to make it simple to provide this "proof of control" without compromising security, and monthly auditing of such was included in the latest S1 amended filing.