GraphicsMagick is a replacement for ImageMagick. Now dev-db/postgis depends only on media-gfx/imagemagick.
Please see also the discussion on a virtual package in the tracker bug.
01 May 2011; Aaron W. Swenson <titanofold@gentoo.org> +postgis-1.5.2-r1.ebuild, +files/postgis_dbs, metadata.xml: Fixes bug 273960, 285082, 296172, 314243, 325113 and 339312