dev-dotnet/gtksourceview-sharp-0.5 depends on >=dev-dotnet/gtk-sharp-1.0 being compiled with USE="gnome" As of gtk-sharp-1.0.4-r1, gnome support is provided through dev-dotnet/gnome-sharp. Reproducible: Always Steps to Reproduce: 1. 2. 3. Simply changing the gtk-sharp dependency to gnome-sharp fixed the problem for me.
*** Bug 73388 has been marked as a duplicate of this bug. ***
I've just commited gtksourceview-sharp-0.5-r1 with the proper deps. Thanks for the report.