I’ve created a custom PCB shape - a simple triangle. It loads fine in Fritzing and looks correct, put when I generate the Gerber file one of the sides are cut off halfway when loaded into a Gerber viewer:
The SVG file (made with Inkscape) is here (links to Github Gist, as svg file counts towards 1 image limit for new users)
It appears to be a result of how the shape was created. I believe you used the star tool and then set the points to 3. I am not sure why this causes a problem but it was relatively easy to fix.
The first thing I tried was tip number 2.1 here Inkscape Tips and Tricks
That did not solve it but I do believe it was needed as you had moved the triangle after putting them in their groups.
The second thing I did was tip number 8 (which i just added) here Inkscape Tips and Tricks and that fixed it.
Here is the one I fixed if you want to compare it.
Sorry for the super late response, but I don’t get a chance to work on my personal projects as often as I would like. I did indeed use the star tool to create the triangle.
Thanks so much, it works great! 
I’ll definitely bookmark your list of tips for future reference.