Next time it would be wiser to post in the Triggers/Scripts section.
Anyways, all GUI code is converted to Jass so it's essentially the same language, GUI is just a frontend for it.
And triggers in GUI are different from triggers in Jass. In Jass they are just another variable type like regions or locations. You can use them to register events with and attach actions and conditions more dynamically then GUI is possible of.
Furthermore, I suggest you go read some Jass tutorials and not use GUI at all.