This is the original MonoGame.Extended.Gui project that was part of the MonoGame.Extended library. This project was removed in favor of adding GUM as the official GUI used by MonoGame.Extended. The original source has been archived here for those that would like to use it.
Please read the blog post at https://www.monogameextended.net/blog/monogame-extended-gum/ for more information.
MonoGame.Extended.Gui is released under the The MIT License (MIT).