Bug 726784 is a tracker for removing gnome-base/libgnomeui. PMASK set today for removal in 30 days. As I understand it, that will make it impossible to emerge gnome-sharp after that.
This applies to 2.24.2-r2, 2.24.4, and 9999 ebuilds.
libgnomeui, libgnome, and libbonoboui have now been tree-cleaned from portage, and they are requred by more than one package in the dotnet overlay.