No it's the other way round, GUI is Jass.
Whenever you trigger something, go to "Edit" (in the Trigger Editor) and click "Convert to Custom Text".
Voila! You've got the Jass version of your GUI...
The thing is that GUI is easier to see as it's graphical and text.
But everything you do in GUI has a very common code in Jass!
So for now, get used to GUI just so that you know how everything functions, then go for Jass... ;D