Let's say I contribute $8,000 to a qualified Roth 401(k), put it into a mutual fund, the value goes to $9,000, I'm 47 years old, the 401(k) is 3 years old, and a do a direct rollover to my Roth IRA. Do I add $9,000 to my Roth IRA basis?
No, only $8,000 gets added to your Roth IRA contribution basis. The Roth 401(k) is not a qualified Roth 401(k) in this case; your example is self-contradictory in this respect.
The $1,000 of gains would only get added to your Roth IRA contribution basis if the distribution from the Roth 401(k) would have been a qualified distribution. For the Roth 401(k) to be a qualified Roth 401(k) you must be age 59½ or over or disabled and the 5-year holding period for this Roth 401(k) is required to have been completed.
