The issue is that while you hold GBTC, they sell off small amounts of the underlying bitcoin holdings to pay expenses of the trust. You are supposed to accept for the profit/loss of the sale of the underlying bitcoin assets in the tax year they sell them, even though you didn't sell the stock itself. If you have held the stock long enough that all underlying sales throughout the year are "long term" capital gains/losses, it is pretty straightforward, and it is really easy if you bought all your GBTC at once and not in several lots on different dates.
Once you figure your profit/loss on the sale of underlying bitcoin and declare that income/loss with the IRS, then you change the basis (eg, original value) of your stock investment by that much. You need to continue to track the remaining basis and the amount of underlying BTC. It is just a pain to do.
