I can only go by what I am told, but if I also read what TDAmeritrade says (on my 1099):
"This transaction represents the sale of assets from a Widely Held Fixed Investment Trust (WHFIT). The cost basis allocation factor is the value of the assets sold divided by the total net asset value of the trust. If you know your cost of the assets sold, use that to determine your gain/loss. Otherwise, determine your cost basis by multiplying your adjusted cost basis by the cost basis allocation factor. For example, if your adjusted basis is $1,000 and the cost basis allocation factor is 0.005 your cost basis allocated to that sale is $1,000 * 0.005 or $5. If there are subsequent sales of trust assets, your adjusted cost basis for the next sale is $995. Sales are reported based on when and for how much the trust sold the asset. This may differ both in timing and amount from what is distributed. There are cases where the proceeds are used to pay expenses and there is no corresponding distribution. For more information refer to regulations section 1.671-5."
This really is highly suggestive that you are required to be adjusting your basis and treating each sale to cover expenses as a capital gain/loss based on the relative price of the underlying assets. So if you want to separate into long- and short-term capital gains, doing it the way GBTC suggests (and not just using the cost basis factor, since you have specific details) allows for that. TDAmeritrade is not suggesting you can just add the whole sale cost to your basis as would have happened with the commission for buying/selling a stock or option.
