Install gtk-sharp2-gapi by entering the following commands in the terminal:
sudo apt update sudo apt install gtk-sharp2-gapi
Description:
C source parser and C# code generator for GObject based APIs
GTK# 2.10 is a CLI (.NET) language binding for the GTK+ 2.10 toolkit . This package includes the parser and code generator used by the GTK# project to parse and bind GObject libraries. Install if you want to bind GObject-based libraries, or need to compile a project that uses it to bind such a library.
Homepage: http://www.mono-project.com/GtkSharp
Version: 2.12.40-2
Section: universe/libs