OnSceneGUI not called: Gizmos#

Editor.OnSceneGUI is called when Gizmos are enabled in the scene view.

Scene view gizmo toggle

The GUI will only show when an appropriate target is selected. If you don't want this behaviour, it may be better to make a custom EditorWindow, a custom EditorTool, or subscribe to the SceneView callback from a method decorated with a InitializeOnLoadMethodAttribute. Each come with their own benefits and drawbacks depending on what behaviour is needed.

OnSceneGUI is still not called.