Hint of yellow and a bit of give to the feel. See link to a good photo below.

Of course you can always pick them earlier than that and let them finish ripening indoors.

Dave

Here is a link that might be useful: Pic of GZ

I am also growing GZ for the first time. But mine does not have any ripe fruits yet. I am anxious to see and taste it.

Be patient with them... they tend to ripen slowly. My experience is that the tastes vary quite a bit from one that is just ripe vs. one that is very yellow and softer.