Since 2013 the Grayscale Bitcoin Trust Fund (GBTC) has offered its investors exposure to Bitcoin (BTC) through a publicly quoted private instrument. However, the trust's convertibility and liquidity vastly differ from an Exchange Traded Fund (ETF).
Trusts are structured as companies, at least in regulatory form, and are 'closed-end funds' which can initially only be sold to accredited investors. This means the number of available shares is limited, and retail traders can only access them via secondary markets. Furthermore, a GBTC share cannot be redeemed for the underlying BTC position.