In /usr/bin/GraphicsMagick-config line 10:
usage='Usage: GraphicsMagick-config [--cflags] [--cppflags] [--exec-prefix] [--ldflags] [--libs] [--prefix] [--version]
      ^-- SC2016: Expressions don't expand in single quotes, use double quotes for that.

For more information:
  https://www.shellcheck.net/wiki/SC2016 -- Expressions don't expand in singl...