Draw Gizmo for selected (list) property?

Is it possible to draw gizmos for selected properties?
For example, in the image below i have an element of a list selected. I would like to draw a gizmo for the outline selected in this list.
At the bottom i included some snippets that show how this specific case is structured. The list contains Outline objects (which do not implement MonoBehaviour). I would assume i have to draw gizmos in the ItemGenerator, but it needs to be able to somehow detect what element in the list is selected.
Is this possible, and how would i have to go about it?

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using UnityEditor;
5.  
6. [CustomEditor(typeof(ItemGenerator))]
7. public class ItemGeneratorEditor : Editor {
8.  
9.     public override void OnInspectorGUI()
10.     {
11.         DrawDefaultInspector();
12.         if (GUILayout.Button ("Generate"))
13.         {
14.             // do stuff
15.         }
16.     }
17. }

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class ItemGenerator: MonoBehaviour {
6.  
7.     public List<Outline> outlines;
8.  
9.     public void Generate()
10.     {
11.         // Generate
12.         // Assign outlines
13.     }
14. }
15.  

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. [System.Serializable]
6. public class Outline {
7.  
8.     private List<Vector3> _points;
9.  
10.     public Outline()
11.     {
12.         _points = new List<Vector3>();
13.     }
14. }