Gtk# is a project to provide a GUI for portable ICS/.net applications, built with either mono or pnet (or Visual Studio.net, or that matter). Since WinForms support isn't there yet in mono, and may never be there for pnet, you pretty much need either this or Qt# for now if you want to do .net on linux. In fact, even when WinForms support is done, you may prefer Gtk# (or Qt#) because you're more comfortable with Gtk+/Glade (Qt/Qdesigner) development than Win32/VS, or because you want a truly free solution.
Gtk# should work with both pnet and mono, but I've only tested with mono, and only building a simple "Hello World" app. I based this package on the Redhat 0.7 package from the Gtk# website; the description seems a little odd to me, but I left it alone. In building Gtk#, I discovered problems with two existing packages, so I've submitted fixes for those as well. The mono-0.23-2mdk package installed broken scripts in /usr/bin/msc and /usr/bin/mbas that pointed into the packager's build root. The problem is that the script that generates these scripts expects to find the real binaries in the install directory, which obviously isn't true when you're packaging. This should probably be fixed properly with the mono people; for now, I just added two lines to the specfile to overwrite these scripts. The gnome-db-0.2.96-6mdk package didn't include pkgconfig support, and wouldn't build without errors (because it installed but did not package the help files). The gtk-sharp package requires my updated mono and gnome-db devel packages to build, but binaries should install just fine with the existing versions. The mono packages don't install the monodoc tools or the mono documentation, so I didn't install the Gtk# docs either. This should probably be changed in the future (or maybe separate monodoc, mono-docs, and gtk-sharp-docs packages would be better?). Someone should package Qt# (especially given Mandrake's slight Qt/KDE preference), but it might be a while before I get around to it (since I've barely even looked at Qt#).
