(which I believe stands for "Just Another Simple Script").
In a recent thread someone pointed out that JASS actually stands for
Just
Another
Scripting
Syntax.
GUI is basically something that covers wraps the actual code into a more readable state
I wouldn't say it's more 'readable', though I would agree it's easier to visualize, since GUI is very verbose and descriptive (in general), compared to some natives which aren't straightforward for novices, e.g. the native to set the tin of a unit is called
SetUnitVertexColor
without a background in art / modeling, I think this would not be clear at first. Also, the last parameter is called "alpha", which corresponds to GUI transparency percent. Again, this is Greek to someone who doesn't know anything about colors in computers, which is probably a majority of mappers.
On the other hand if we need a complex algorithm, I'd probably want to read it in JASS or vJASS rather than GUI.
its scripting language, not programming
I am guessing the distinction is the purpose of the language? Otherwise I believe JASS is Turing complete? Like LaTeX, which isn't a programming language but a typsetting one, which is also Turing complete if I recall correctly. Just me clarifying your meaning so I understand better, since I'm not a programmer.
Which provides more efficience ? What type of things use these... systems
Ultimately you will want to do your map script in JASS or vJASS or even Wurst if you want to
a) produce high quality, object oriented code, which is modular + easy to change
b) use version control (SVN, github, etc.), which will save you from making bad changes (you can rollback instantly). Also, if your project has more than one developer, it will be very easy to collaborate and work together on the code, AFAIK any serious code / script development is under version control.
c) Have full control over your map script--I believe there are some things which are difficult or impossible to do in GUI which can be done in JASS.
I would recommend doing GUI if you're just making something for the fun of it, which is why GUI was designed the way it is, but learn vJASS or Wurst if you want to be a serious mapper.
Resource-wise, there are many resources for both vJASS, JASS, and GUI, but you will find the most refined resources are made in vJASS. Wurst does not have as large of a community as vJASS (given its age), and so far less resources.