You should build it from source (you should make sure that your deb-src lines in your sources.list are activated, and you ran sudo apt-get update):

mkdir src && cd src && apt-get source graphicsmagick

Enter the graphicsmagick-* directory. Now, run vim debian/rules (if you want to use another text editor, be my guest) and look for the line that reads:

./configure $(gm_confflags)

bellow this add (you need to press I to insert) --with-quantum-depth=16 so it should more or less looks like:

    CFLAGS="$(CFLAGS)" CXXFLAGS="$(CFLAGS)" LDFLAGS="$(LDFLAGS)" 

        ./configure $(gm_confflags) 

            --with-quantum-depth=16 

            --enable-shared 

            --enable-static 

            --enable-libtool-verbose 

            --prefix=/usr 

            --mandir=$${prefix}/share/man 

            --infodir=$${prefix}/share/info 

            --docdir=$${prefix}/share/doc/graphicsmagick 

            --with-gs-font-dir=/usr/share/fonts/type1/gsfonts 

            --with-x 

            --x-includes=/usr/include/X11 

            --x-libraries=/usr/lib/X11 

            --without-dps 

            --without-modules 

            --without-frozenpaths 

            --with-perl 

            --with-perl-options="INSTALLDIRS=vendor"

Save the file (press Esc, then type :wq and hit Enter), and run:

sudo apt-get build-dep graphicsmagick

dpkg-buildpackage

sudo dpkg -i ../graphicsmagick*.deb

The first one installs the build dependencies, the second builds the package, the third installs the packages. And thats it.