David describes what black belt means here:
I believe the general thinking is that it takes about two to three years to get to black belt with this methodology. However Wikipedia describes black belt as follows:
'The shodan black belt is not the end of training but rather as a beginning to advanced learning: the individual now "knows how to walk" and may thus begin the "journey".'
So I suppose this means what do you mean by master GTD? GTD is like an onion (or a parfait). There are a lot of layers.
In addition to David's list of belt descriptions there is probably at least one additional belt:
Black Belt - 3rd Degree: Your next actions and projects are in alignment with your higher altitudes in support of your life mission.