Digging a little deeper into this, I /think/ this is a bug in the GraphicsMagick source file http://hg.code.sf.net/p/graphicsmagick/code/file/tip/config/type-ghostscript.mgk.in which hard-codes the font file names and just makes the path configurable.
I added the attached patch file to the Debian patches and re-build the package, which now processes SVG files as expected, so this seems to be a fix. Also attached is the *very* ugly Python script I used to extract the URW font paths from the Ghostscript config and to modify type-ghostscript.mgk.in - maybe it is helpful. Thanks, Albrecht.
font-urw35-issue-1019717.patch.gz
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fix-gm-fonts.py.gz
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pgpwh8glYK_BO.pgp
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