Making it display a Gui

you like ellipses...

Is Inventory a button or a type?

Input.GetButton() takes a String defining a button configured in your input manager.

If you have defined a button Inventory, then you probably wanted

Input.GetButton("Inventory")

If you want the code to only act when the button is held down, you probably wanted

Input.GetButtonDown()

If you wanted the inventory thing to only happen when the Input.Get... was true, then you should put it inside of an if statement

if(Input.GetButtonDown("Inventory"))
    //show your GUI

What is show supposed to be? If show is an enum, class or class instance, then this is syntactically correct, but will not actually do anything.

What is inventoryGui supposed to be? If it is a variable of a class, then you should be assigning or changing it like show.inventoryGui = something;. If it is an enum, then you should be assigning something to have its value like something = show.inventoryGui;. If it is a function, you must call it with parentheses like show.inventoryGui();.

1. var button : boolean;

   1. function Update()

   2. {

   3. //if user presses Tab then the GUI should appear;

   4. if (Input.GetKeyDown(KeyCode.Tab))

   5. {

   6. button = true;

   7. }

   8. //if he's not pressing Tab anymore let's make the GUI dissapear;

   9. if (Input.GetKeyUp(KeyCode.Tab))

   10. {

   11. button = false;

   12. }

   13. }

   14. //now let's make our GUI;

   15. function OnGUI()

   16. {

   17. if (button == true)

   18. {

   19. // you can change the values and the text displayed as you wish;

   20. GUI.Box (Rect (100,100,100,100), "it works" );

   21. }

   22. }