Surely the component type name would be Text

To find doco on the UI (some people still say "new" UI, but it is years old now)

https://unity3d.com/learn/tutorials/topics/user-interface-ui

surprisingly the Unity doco is quite good on this topic.

Gah, had written a nice reply then realised you were using Javascript rather than C#. Makes the answer somewhat simpler:

 gui.GetComponent(GUIText).text = "Score: " + CurrentScore;

Basically, remove the " so you just pass in the type of object you're expecting to get a result about, not a string.

Personally I never use Javascript so that might not work, in which care you might need something like

 function Update()
 {
 var myGuiText : GUIText;
 myGuiText = gui.GetComponent(GUIText);
 myGuiText.text = "Whatever";
 }

That's not really a very elegant answer as there are no checks to make sure that myGuiText is actually an object (might return null if GUIText doesn't exist as a component on your object) and calling GetComponent every frame is slow.

Personally, I'd recommend looking at switching to C#. It's a little more intimidating if you're getting started but you'll be better off in the long run (most examples etc focus on C# too).

As Fattie said too, the newer UI system is the way to go. It's far more efficient to build and execute and I imagine there are plenty of samples around.