If I rolled over a traditional 401(k) into a Roth 401(k), is the correct 1099-R code "G?" If not, which is the correct code to indicate this should be taxed?

Code G is for a direct rollover of a distribution to a qualified plan, a section 403(b) plan, a governmental section 457(b) plan, or an IRA.  It does apply to a 401k to Roth 401k. 

You will want to enter the 1099-R as you received it because this is what is reported to the IRS.    In TurboTax there will be a follow up question so that you can indicate the conversion.  (See below)

 @shannon-thielen394

[Edited 02/08/25 | 10:22 am PST]

MaryK4's original reply is entirely incorrect.  Code G is the correct code for your in-plan Roth rollover (which is a type of direct rollover).

The amount that the payer reported in box 2a is the taxable amount.  Enter the Form 1099-R exactly as received.  On the page that follows, indicate that you moved the funds to a Roth 401(k) and TurboTax will include the taxable amount on Form 1040 line 5b with the ROLLOVER notation.