GUITexture is actually a component, so you need to do (GUITexture)myGameObject.GetComponent(typeof(GUITexture)) (in C#). I can't remember, but there might be myGameObeject.guiTexture too.
Probably better to do this in two steps so that your code won't crash if the object isn't found:
GameObject go = GameObject.Find("GUI_LeftArrow");
if ( go != null )
BtnArrow = go.guiTexture;
Other alternatives: you can walk all objects in the scene of a type by calling FindObjectsOfType, or if there is only one of that type, you could call FindObjectOfType.
Note: above code is not tested, but it should be pretty close.