Code G is a tax-free rollover. This would have been incorrect by the issuer, but the issuer may not have known it was a conversion, not a rollover. Depending upon their age I would change both the code and the taxable amount on the 1099-R worksheet to Code 2 or 7, fully taxable, and then in the lower section of the worksheet mark it as all converted to Roth.

If the transaction was with the same investment house they should have done this correctly. If from one to another, it's your responsibility to report properly even if the form itself is not correct.