net-im/gabber-0.8.7-r1 does not ebuild without gnomemm>=1.1.12 emerging gnomemm first works fine.
gabber-0.8.7-r1 already has a >=gnome-extra/gnomemm-1.2.0 dependence in its ebuild. So that emerging gabber 0.8.7-r1 should also grab the need gnomemm dep for you (if you're missing it). However, just checkout the new revision: gabber-0.8.7-r2