/usr/include/GraphicsMagick/ is in libgraphicsmagick++1-dev 1.3.30+hg15796-1~deb9u2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/GraphicsMagick/Magick++/
/usr/include/GraphicsMagick/Magick++.h
/usr/include/GraphicsMagick/Magick++/Blob.h
/usr/include/GraphicsMagick/Magick++/CoderInfo.h
/usr/include/GraphicsMagick/Magick++/Color.h
/usr/include/GraphicsMagick/Magick++/Drawable.h
/usr/include/GraphicsMagick/Magick++/Exception.h
/usr/include/GraphicsMagick/Magick++/Geometry.h
/usr/include/GraphicsMagick/Magick++/Image.h
/usr/include/GraphicsMagick/Magick++/Include.h
/usr/include/GraphicsMagick/Magick++/Montage.h
/usr/include/GraphicsMagick/Magick++/Pixels.h
/usr/include/GraphicsMagick/Magick++/STL.h
/usr/include/GraphicsMagick/Magick++/TypeMetric.h