$ sudo apt-get update
$ sudo apt-get install libgtk-dotnet3.0-cil 

libgtk-dotnet3.0-cil is installed in your system.

Make ensure the libgtk-dotnet3.0-cil package were installed using the commands given below,

$ sudo dpkg-query -l | grep libgtk-dotnet3.0-cil *

You will get with libgtk-dotnet3.0-cil package name, version, architecture and description in a table

libgtk-dotnet3.0-cil package basic information: