GizmoIcon.cs

using UnityEngine;
using System.Collections;

/// <summary>
/// This was just a quick-and-dirty tool when inside the Unity Editor.
/// Need an Icon on something? Throw this on it, assign a texture (via URI string), done.
/// </summary>
public class GizmoIcon : MonoBehaviour {

    public string IconFile;

    void OnDrawGizmos()
    {
        Gizmos.DrawIcon(transform.position, IconFile, true);
    }

#if !UNITY_EDITOR
	// If we're not in the editor, this module shouldn't exist.
	void Awake () {
	    Destroy(this);
	}
#endif

}

In order to make a gizmo appear, the file has to be in Assets/Gizmos

https://docs.unity3d.com/ScriptReference/Gizmos.DrawIcon.html