I don't think that changes it being flat.
Capital gains changes the height of the curve, but not the fundamental shape. See Figure 5.
Curve will still be flat for individuals, and not households. Besides, capital gains only affects income when you sell something, which is probably why Census doesn't measure it. It's like measuring middle class income based on them selling their house.
Best I can see, that CBO paper addresses only households. Look up "individual GINI index" or an index for "all people."
P.S. Wikipedia is a perfectly good source for finding better sources.
