You should post the code you already have made to give people an idea on exactly what you are trying to do.
Generally if you want something to happen on clicking something in the inventory that isn't a button, using mouse event controls is a good idea. If you want this GUI to be able to manipulate your regular tool inventory, then you are going to need both a server sided portion and a client sided portion. I recommend looking at the add item event script on RTB for a reference on how to manipulate the tool inventory from the server side.
