GraphicsMagick CgBI unknown critical chunk

I am using GraphicsMagick and currently running into the following issue:

when executing

gm identify <filename>.png

I'm getting the following error:

gm identify: CgBI: unknown critical chunk (<filename>.png)
gm identify: Request did not return an image.

This is a png extracted from an iPhone. Though it may not contain an actual image, I do need the image info.

Tried opening it with several editors:

1. The png does open in Paint with the right height x width but as completely white.
2. The png does not open in GIMP, saying it might be corrupted
3. The png does open in Paint.net with the right height x width as a transparent image

Using the debug parameter provides the following line:

libpng-1.5.14 error: CgBI: unknown critical chunk

I'm running the latest GraphicsMagick (1.3.18).

Does anyone know what's going on, and how I will be able to get this to return the image info?

Thanks in advance.