Install gtk-sharp3 by entering the following commands in the terminal:
sudo apt update sudo apt install gtk-sharp3
Description:
GTK# 3.0 suite, CLI bindings for GTK+
GTK# 3.0 is a CLI (.NET) language binding for the GTK+ 3.0 toolkit and related libraries. . gtk-sharp3 is a metapackage containing dependencies for the GTK# 3.0 suite.
Homepage: http://www.mono-project.com/GtkSharp
Version: 2.99.3-2
Section: universe/cli-mono