The GraphicsMagick-config
script is a separate program rather than a pkg-config resource, which explains the problem.
Also, running pkg-config with a non-option argument seems to fail without printing an error message, which might have confused you.
In addition to this script though, the library does install a GraphicsMagick.pc
data file for pkg-config. So you should be able to get your code running with:
#cgo pkg-config: GraphicsMagick