Import the GDI package in Unity by clicking Assets -> Import Package -> Custom Package. Select the package in the popup and import all contained files.
Your projects 'Asset' folder should now contain an folder called 'Editor' containing the file GDIWindow. In the main menu of Unity select Window -> GDI to open the editor window.
In the editor window click Graph -> Open to open one of the existing examples or Graph -> New to create a new Graph.
|Middle mouse button + drag||Drag the canvas.|
|Scroll||Zoom the canvas.|
|Right click on canvas||Open node adding conext menu.|
|Right click on node||Open node conext menu.|
|Left click on node||Select node.|
|Left click on node + drag||Move node.|
|Left click on canvas + drag||Selection box.|
|Left click on node + shift||Add node to selection.|
|Left click on edge||Add edge path point.|
|Drag edge path point||Move egde path point.|
|Right click on edge path point||Remove point.|
|Press 0||Reset zoom.|
To start using the editor with your script take a look at the folder Assets/GDI/Behaviour/. Simply add a behavior as a component to a GameObject in the scene. Take a look at the source code of the behaviour and read its documentation.