You don't enter gains and losses into TurboTax. You enter the proceeds and basis for each sale or group of sales, and TurboTax calculates the gain or loss. If you enter the proceeds and basis accurately in TurboTax for all of the sales, the calculated gain or loss will be accurate.
The brokerage does not report gains or losses to the IRS. It reports the proceeds, and may or may not report the basis for any particular sale.
