If I'm not mistaken, I've seen this before--it's a PSD (Photoshop) file --the individual elements are meant to be used as different fantasy backgrounds. You're interested in the inventory section?
You can add that an Inventory screen as a sub menu screen in your menus. Look at the tutorial-menuscreens.
The whole background of the inventory part could be used as the box in your guibox for your inventory screen and the small slot type boxes would probably be backgrounds for your items.
Would suggest following the tutorial first to set up the menu screens and THEN alter them to fit what you want. That's the way I did it.
If you want the player's level and attack attributes to show up in the inventory screen you might be able to but not sure why you'd want to show this in an Inventory screen.
There is a text code for the amount of money the player has in inventory--look at the tutorial for the overview screen. #currency