Nugget hunting is very time consuming and frustrating if you have never done it. Combined with the limited areas here in Ca. to hunt you may find that the parks produce much better.
If you really want to try it, I would buy either a GMT or MXT. The GMT is the superior gold detector. Way ahead of the GMZ. And a touch better then the MXT. It will afford you a greater chance for success over the GMZ.
I think you would be better off buying an MXT to start with.
This is my reasoning.
There is a very strong chance you may not like nugget hunting or find you don't have sufficient time to put into it. The MXT is ALMOST as good at finding raw gold as the GMT. If nothing else, it will find you larger nuggets deeper. You have a greater variety of coils to match the terrain and ground conditions with and they will work with your M6
If for any reason you decide nugget hunting is not for you, you can either sell the MXT much easier then the GMT or use it as a second detector. You already know how to use it, it is no different then your M6.
By the way, although your M6 isn't set up for nugget shooting, it is capable of doing it.