Code G with the IRA/SEP/SIMPLE box is correct for a direct rollover from a traditional IRA to the traditional account in a 401(k).
Ignore TurboTax's bogus message that suggests that this has anything to do with a Roth IRA. Whoever programmed that didn't know what they were doing.
This rollover is indeed a distribution from the IRA, albeit a nontaxable one.
