dev-dotnet/gtk-sharp itself depends on x11-libs/gtk+:2 which is EOL. No revdeps.
It appears that Fedora are able to use GTK3/gtk-sharp-3 with some easy hack: https://src.fedoraproject.org/rpms/libgpod/blob/rawhide/f/libgpod.spec Of course, no such thing as gtk-sharp:3 in tree yet.
Dependency dropped in commit 836056021e23a3b92c646050d8d91adb3eb451d4.