GS need a big man, green and horford are not versatile, they need a big one to defend guy like wemby or Chet. Kuminga is not in Kerr's plans, so replace him by gafford and a good 3 and D like marshall is good.

For Mavs, they have a lot of guy on 4/5 (davis, flagg, lively, gafford, whashington, Powell). so trade gafford can seems a big loss but they add a big scorer with kuminga next to flagg for the frontcourt. Moreover, they keep depth with trayce jackson-davis.