You don't need to buy anything (except WarcraftIII [with extension for make spells and some others things]) for map.
GUI is just the Graphic User Interface which is an interface of the JASS. All triggers are basically in GUI in the editor.
JASS (Just Another Scripting System) is just the coding language used by the World Editor; for acceed to the Jass, you need to convert a trigger into "Custom Script".
These 2 things are included in the World editor and you don't need to download anything.
After, for use vJass which is an extension of the Jass, you need to use the JNPG which is free to download.