There are 2 box 21s on the TurboTax T3 entry page.
You would enter your Foreign capital gains (if any) in the first one. In the second box 21, you would enter your Total capital gains in the second box, including your Foreign capital gains.
If you only enter something in the 1st box 21, you will get an error message, because the 2nd box 21 must be at least the amount in the 1st box.
If you don't have any Foreign capital gains, then you would leave that box at 0.00, and enter your amount in the 2nd box 21.