The gui.cs organization hosts projects related to gui.cs that Miguel de Icaza wrote for mono-curses in 2007. The org is now maintained by Tig Kindel.
The original gui.cs was a single file and tied to the curses library. Since then, gui.cs has been transformed into Terminal.Gui, a full-featured library for building modern, cross-platform, TUI applications.
A presentation of gui.cs was part of the Retro.NET talk at .NET Conf 2018 Slides
The other projects in this organization are either dependencies of Terminal.Gui or related offshoots.
The maintainers welcome contributions from the community.