There are packages in portage that can/should depend (or rdepend) on either imagemagick or graphicsmagick, not only on imagemagick.

  Packages that depend on imagemagick only:
  (not sure if all of them can actually depend on graphicsmagic)

app-admin/recursos
app-editors/kile
app-emacs/thumbs
app-emulation/playonlinux
app-misc/bins
app-misc/booh
app-misc/matew
app-misc/tracker
app-mobilephone/obex-data-server
app-office/eqe
app-office/karbon
app-office/kchart
app-office/koffice-data
app-office/koffice-libs
app-office/kplato
app-office/kpresenter
app-office/krita
app-office/kspread
app-office/kword
app-office/lyx
app-office/multitalk
app-office/texmacs
app-text/calibre
app-text/cuneiform
app-text/pdf2html
app-text/pdf2oo
dev-embedded/ponyprog
dev-lang/gdl
dev-php5/magickwand
dev-php5/pecl-imagick
dev-db/postgis
dev-python/pythonmagick
dev-ruby/gruff
dev-ruby/mini_magick
dev-ruby/rmagick
dev-tex/html2latex
dev-tex/latex2rtf
dev-tex/tex4ht
games-board/cgoban
games-strategy/wesnoth
gnome-extra/libgsf
kde-base/kopete
kde-misc/kim4
mail-filter/spamassassin-fuzzyocr
media-gfx/album
media-gfx/asymptote
media-gfx/autotrace
media-gfx/fbida
media-gfx/freewrl
media-gfx/greycstoration
media-gfx/igal
media-gfx/inkscape
media-gfx/jigl
media-gfx/k3d
media-gfx/llgal
media-gfx/mkgallery
media-gfx/recoverjpeg
media-gfx/pstoedit
media-gfx/smile
media-gfx/zphoto
media-libs/gegl
media-libs/mlt
media-libs/vips
media-libs/xine-lib
media-plugins/kipi-plugins
media-plugins/vdr-coverviewer
media-plugins/vdr-graphtft
media-plugins/vdr-mp3ng
media-plugins/vdr-music
media-plugins/vdr-picselshow
media-plugins/vdr-skinelchi
media-plugins/vdr-skinenigmang
media-plugins/vdr-text2skin
media-sound/codecgraph
media-sound/frescobaldi
media-sound/gejengel
media-sound/gpodder
media-sound/kwave
media-tv/tvmovie2vdr
media-video/chaplin
media-video/dvdauthor
media-video/dvdrip
media-video/dvd-slideshow
media-video/lives
media-video/motiontrack
media-video/noad
media-video/oxine
media-video/tovid
media-video/transcode
media-video/videotrans
net-analyzer/scapy
net-mail/libpst
net-misc/plowshare
net-misc/secpanel
net-p2p/gift
net-print/apsfilter
net-zope/photo
rox-extra/magickthumbnail
sci-chemistry/chemical-mime-data
sci-chemistry/xdsi
sci-geosciences/openstreetmap-icons
sci-visualization/gri
sci-visualization/opendx
sys-apps/paludis
sys-fs/flickrfs
www-apps/bugzilla
www-apps/coppermine
www-apps/curator
www-apps/gallery
www-apps/mediawiki
www-apps/mypictures
www-apps/redmine
www-plugins/moonlight
x11-misc/rss-glx
x11-misc/shutter
x11-misc/slim
x11-misc/xnee
x11-misc/xrmap
x11-plugins/gkrellshoot
x11-plugins/pidgin-latex
x11-plugins/wmgrabimage
x11-themes/fvwm-crystal
x11-themes/fvwm-themes
x11-themes/nuvox
x11-themes/tangerine-icon-theme
x11-themes/tango-icon-theme
x11-themes/tango-icon-theme-extras
x11-wm/awesome
xfce-extra/thunar-thumbnailers

  packages that depend on either imagemagick or graphicsmagick:

sci-visualization/pyxplot

  packages that depend on graphicsmagick only:

none