How much to tax from whom by what means: these are matters of opinion, as Gates says. Tax labor less, consumption more, and eliminate subsidy granted by silly tax breaks (such as 'carried interest') for the earnings of capital, especially finance capital.
What to do is the core of these kinds of discussions, just as it is with pollution impacts on climate. Exactly what is going on is not relevant.
Piketty is clueless about capital, except as to the point Mr. Gates restates: inequality is a NECESSARY part of capital and of progress by any means, inescapably so. But social cohesion is also, and at some point, inequality can undermine social cohesion, just as boneheaded support for cohesion can suppress progress.