GUIDebug.cs

public class GUIDebug : Monobehaviour
{
    public static Bounds GetRectTransformBounds(RectTransform transform)
    {
        Vector3[] WorldCorners = new Vector3[4];
        transform.GetWorldCorners(WorldCorners);
        Bounds bounds = new Bounds(WorldCorners[0], Vector3.zero);
        for(int i = 1; i < 4; ++i)
        {
            bounds.Encapsulate(WorldCorners[i]);
        }
        return bounds;
    }
    
    private static Texture2D MakeTexture(float opacity)
    {
        Texture2D texture = new Texture2D(2, 2, TextureFormat.ARGB32, false, true);
        texture.SetPixel(0, 0, new Color(1, 0, 0, opacity));
        texture.SetPixel(0, 1, new Color(0, 1, 0, opacity));
        texture.SetPixel(1, 0, new Color(0, 0, 1, opacity));
        texture.SetPixel(1, 1, new Color(1, 1, 1, opacity));
        texture.Apply();
        texture.filterMode = FilterMode.Point;
        return texture;
    }
    
    public float opacity = 0.5f;
    private Texture2D tex = null;
 
    private void OnValidate() => tex = MakeTexture(opacity);
    private void Awake() => tex = MakeTexture(opacity);
 
    private void OnDrawGizmos()
    {
        Bounds bounds = GetRectTransformBounds(transform as RectTransform);
        Rect screenRect = new Rect(bounds.min, bounds.size);
        Gizmos.DrawGUITexture(screenRect, tex);
    }
}