Gliding a GUI Element in...

Here's a Unity thread about Lerping a GUI element .. compliments of Google: Animating GUI movement?

To make things even easier you could use the animation engine LeanTween and accomplish this with very little code:

 public var grumpy:Texture2D;
 private var grumpyRect:LTRect;
 
 function Start () {
     grumpyRect = new LTRect( -grumpy.width, 0.5*Screen.height - grumpy.height/2.0, grumpy.width, grumpy.height ); 
 
     // Slide in
     LeanTween.move(grumpyRect, new Vector2(0.5*Screen.width - grumpy.width/2.0, grumpyRect.rect.y ), 1.0, {"ease":LeanTween.easeOutQuad});
 }
 
 function OnGUI(){
     GUI.DrawTexture( grumpyRect.rect, grumpy);
 }

There are more advanced examples here.