Show 2D Polygon Collider as Gizmo, also in Edit Mode (Not support rotation)

showPolygon2Dcollider.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;


[ExecuteInEditMode]
public class VisibleCollider : MonoBehaviour {
	
	public bool showLine = true;
	public Color lineColor = Color.blue;

	private PolygonCollider2D[] thisColliders;
	private List<Vector2[]> pointsList;
	private List<Vector3> _tList;
	
	void OnDrawGizmos() {

		if (!showLine) {
			return;
		}

		pointsList = new List<Vector2[]>();
		_tList = new List<Vector3>();
		
		thisColliders = gameObject.GetComponents<PolygonCollider2D>();
		foreach (PolygonCollider2D c in thisColliders) {
			
			Vector2[] pointWithOffset = new Vector2[c.points.Length];
			
			for (int i = 0; i < c.points.Length; i++) {
				pointWithOffset[i] = c.points[i] + c.offset;
			}
			
			pointsList.Add(pointWithOffset);
			_tList.Add(c.transform.position);
		}
		
		for (int j = 0; j < pointsList.Count; j++) {
			
			Vector2[] points = pointsList[j];
			Vector3 _t = _tList[j];
			
			Gizmos.color = lineColor;
			
			for (int i = 0; i < points.Length - 1; i++) {
				Gizmos.DrawLine (new Vector3 (points [i].x + _t.x, points [i].y + _t.y), new Vector3 (points [i + 1].x + _t.x, points [i + 1].y + _t.y));
			}

			Gizmos.DrawLine(new Vector3 (points [points.Length - 1].x + _t.x, points[points.Length - 1].y + _t.y), new Vector3 (points [0].x + _t.x, points [0].y + _t.y));
			
		}
	}
	
}

You can account for scale and rotation by
Gizmos.matrix = transform.localToWorldMatrix;

and removing
private List _tList;

so that the position offset isn't applied twice