JASS is just another way to express detailed on coding
Basically, GUI = JASS, but GUI just show you the true meaning of a function, but when it is converted to coding, GUI is just basically a JASS version, only understandable words appear, unlike JASS which requires you a special knowledge to understand them (but easy to understand the function)
That is why GUI has this Custom script action which is used to call functions in JASS
What GUI can do , JASS can do
What JASS can do , sometimes GUI can't do
If you master the Custom script, GUI can also become nearly as efficient as JASS, trust me
Because technically, Custom script = JASS