The best single sentence description, IMHO, is that GT (Glamorous Toolkit) is the current manifestation, implementation, proof of concept of the idea that making systems explainable through mouldable development is very powerful.
Furthermore it is a very nice tool in its own right (again IMHO).
Saying 'GT is x' where x is any of the subjects in the text is problematic. Most people will cling to one perspective, the one they are most familiar with or like the most, and mostly forget the other ones.
There is lots of stuff to read or watch:
- https://blog.feenk.com/ - https://book.gtoolkit.com
(BTW, everything you see in the above two web sites is actually written inside GT, notebook style with active content).
But reading about it is no substitute to trying it out and experiencing it yourself, though there is a learning curve for sure.