Code G in box 7 with the IRA/SEP/SIMPLE box unmarked and a nonzero amount in box 2a implies that at least some portion of the pre-tax part of the distribution that was rolled over to a Roth IRA (or to a designated Roth account in the same retirement plan as the plan from which the distribution was made). If you indicate that the entire amount was rolled over to a Roth IRA, the entire gross distribution instead of just the amount in box 2a will be treated as taxable.
Did you do a split rollover of a distribution from a traditional account in an employer's qualified retirement plan to a combination of traditional and Roth IRAs or did you roll the entire amount over to a Roth IRA?