Given correct entry of this Form 1099-R, TurboTax will correctly calculate the MAGI for the purpose of the limitation on Roth IRA contributions.
Code G with the IRA/SEP/SIMPLE box unmarked and a taxable amount in box 2a can mean a either a direct rollover to a Roth IRA or an In-plan Roth Rollover (IRR) to a designated Roth account in an employer-provided plan. You must tell TurboTax which type of rollover was done. Only a rollover to a Roth IRA is permitted to be subtracted from AGI for this purpose, subtraction of the taxable amount of an IRR is not.
