Intuit just clarified that the Gain/Loss csv is no longer supported. To enter transactions from arbitrary sources use instead the Universal Template csv.
The problem is that the instructions to upload the filled in template do not match the TurboTax UI and there's just no way to upload it. The actual path closest to the instructions is Wages & Income > Investment and Savings > Stocks, Cryptocurrency, Mutual Funds, Bonds, Other (1099-B, 1099-DA) > Add investments > Enter a different way > Upload. But that doesn't support csv files.
