All it is is a GUI window with your inventory items listed in it. It's not exactly something new is it. Like trix said, you should study kavan's code and add some extra features to it. Also, you could make your GUI look more visually appealing. It looks simple and horrible.
Almost every good inventory system has some sort of category system. Even the default inventory has one. If you don't categorise your items and you have a lot of weapons, you will find locating a specific one pretty hard. It's a nice, but very very basic code you have there.