One does not simply make a GUI and expect it to magically work wherever you want it too.
This.
In order for someone to make a GUI that displays guns, prices, and has the ability to buy these items, you would require a mod that has a money system, and has preferences for changing price, item, and picture.
What would be good is the ability to display VCE variables in GUIs, thus making for a more complex and design-oriented RPG. The displaying of players' scores is possible, so I'd imagine if someone could figure out a way to store the saved VCE data into a format that is similar to players' scores, then we would be able to display this stuff. If they did this, then your suggestion for a Store GUI would work very easily.