Code G is the correct code, so there appears to be nothing wrong with the Form 1099-R that you received. Make sure that in the questions that follow the page where you enter the form details that you indicate that the money was rolled over to a Roth IRA (not to a Roth 401(k)) and the amount that you indicate as the amount of after-contributions you made is box 1 minus the amount in box 2a (or answer No if box 2a equals box 1). If any basis in after-tax contributions was included in this distribution, that amount should be present in box 5 of the Form 1099-R provided by the TSP.
