Perhaps management fees are not the right word, but they are not sales fees either. According to the tax attorney I spoke with, these are the fees for holding/managing the bitcoin. They are not "sales fees" that we can use to directly offset the cost of our buying/selling. They are the fees charged to "hold" the bitcoin, sort of like the safety deposit fee that you might have to pay to store gold, I suppose. To state it more directly, these aren't fees to sell the bitcoin, they are the value of the bitcoin they sold to run/manage the trust.
The reason GBTC gives you all the information is that it is that effectively the IRS views the BTC they hold as the asset you purchased, not just the "stock" itself per se. So when they have to sell off BTC, you must report the capital gain (or loss) on that BTC that was sold. Their complicated tables give you a way to do that. They tell you the fractional share of bitcoin they were holding that were sold and the $ each of those were worth. You subtract that from the bitcoins you "controlled" when you bought GBTC (they tell you how to calculate that too). You adjust your basis on that initial purchase based on the capital gain/loss that you took, and you recalculate your ratio of remaining basis and remaining controlled bitcoin at the end of each year. That way, when you finally do sell all your shares, you can adjust the basis for the shares (initial purchase) so that you sale of the "stock" accurately reflects the gain/loss since some of the underlying assets have been sold off in between.
