The ebuild has automagic deps on gtk+ and gimp (plus the gtk+ dependency is incorrect, needs gtk+-2). Patch attached; maybe it should be USE=gimp instead of USE=gtk; USE="-gtk" now disables the gimp plugin and libgutenprintui2 altogether).
Created attachment 117220 [details, diff] gimp-print-5.1.0.ebuild.diff
thank you. I have committed this!