I’ve only found this out a few days ago, but if you export your part in Fritzing’s Production Gerber Export it shows errors.
Basically Copper1 is missing, so you need to make a full duplicate copy of Copper0 and call it Copper1, and then put it inside Copper0
In Inkscape I open the XML editor and just duplicate Copper0, rename Copper1, and just indent it into Copper0.
There are no instructions for this stuff so I had to reverse engineer it from other parts, but one guy made group Layer1, then in it Silkscreen group with all it’s nodes, and then an empty of nodes Copper1, and in that he put Copper0 with all the nodes, and that passes the Gerber test.
Other things I have found out is that if the part body needs extra holes, like mounting holes, these holes have to be put in the Copper groups or they won’t get drilled.