Mike--
I grew GZ last year, and personally found it to be at its best just right after some of the striping turned a light amber, and of course, when the tomato actually felt ripe to me.
I had tried letting fruits set a bit longer to see what happened as to coloration and taste, and to my tastebuds they just seemed mushy at that point.
Also, the color did not improve that all that much in regards to getting that dramatic contrast in the striping like one sees in some pics/some climates. Just my experience.
Here's some pics of GZ from last year along w/CP and Rutgers right at what I considered to be the ripe stage, but your tastebuds may vary (click to enlarge)--
