Actually, the issue here is that you did not sell any shares of GBTC (or ETHE or ETHC or LTCN as the case may be) but the GBTC trust itself, in order to pay its expenses, sold Bitcoin (or Ethereum or Ethereum Classic or Litecoin as the case may be) tokens and those expenses and sales pass through to you. So the number of shares that you have of GBTC remains the same, but you are on the hook for reporting these Bitcoin sales.
The net effect is that you have to calculate:
1) the total expenses that pass through to you (which happens to be equal to the sales price of the Bitcoin that was liquidated on your behalf to cover those expenses)
2) the number of tokens that were sold to cover those expenses
3) your cost basis in those tokens
If you are in a position under current tax law to deduct your investment expenses, then you can take (1) as a deduction. In any case, you need to report the Bitcoin sales on Schedule D with sales price = (1) and cost basis = (3).
For detailed instructions on how to do all this, consult the tax reporting document on Grayscale's site at https://grayscale.com/tax-documents/.
And, yes, for those who bought GBTC thinking that it was a hassle-free way to own an interest in some Bitcoin, this is all a gigantic rude awakening.
