If you initially purchased your GBTC more than a year before the first sale (so like prior to Jan 2021) then you can be assured that all of the BTC sales were long term. In that case, it is easy, just do that multiple as you said. My 1099 actually has TD Ameritrade listing each and every individual sale, so I need to make that wash, but if you are putting it in by hand, you could probably get away with the total for the year that was so long as all of the BTC sales were long term capital gains/losses.
