Well there's no built in way to detect this. You have to check it manually. When you use a GUI window or group / area, you can use its Rect to check if the mouse is inside.

// C#

Rect windowPos = new Rect(10,10,200,150);

void OnGUI()
{
    Event e = Event.current;
    windowPos = GUI.Window(0, windowPos, drawWindow, "MyWindow");

    if (e.type == EventType.MouseDown && !windowPos.Contains(e.mousePosition))
    {
        // Click was outside of the GUI window
    }
}

void drawWindow(int aID)
{
    // draw the window content
}

Here's the same thing in UnityScript.

// UnityScript    
var windowPos = new Rect(10,10,200,150);

function OnGUI()
{
    var e = Event.current;
    windowPos = GUI.Window(0, windowPos, drawWindow, "MyWindow");

    if (e.type == EventType.MouseDown && !windowPos.Contains(e.mousePosition))
    {
        // Click was outside of the GUI window
    }
}

function drawWindow(ID : int)
{
    // draw the window content
}

This is the closest as it gets so far:

public function Update() {
if(Input.GetMouseButtonDown(0) && !MouseOver()) {
Close();
> }
}
public function MouseOver() { var x = Input.mousePosition.x; var y = Screen.height - Input.mousePosition.y; if(x >= SizeAndPos[0] && x <= (SizeAndPos[0] + SizeAndPos[2]) && y >= SizeAndPos[1] && y <= (SizeAndPos[1] + SizeAndPos[3])) return true; return false; }

I get the mouse coordinates X and Y, and check if they are in the GUI's boundaries, I guess this is the most efficient as it gets in GUIs. I wasn't able to use

windowPos.Contains(e.mousePosition)

because of that error i commented above.

Hope this helps someone with the same question in the future.

Thanks