I just quickly downloaded and tried it. It seems like Pharo Smalltalk with a notebook interface, basically a new UI, but Pharo is still there.
If I had time today I would take my old natural language processing Pharo class library and move it over to GT. There are many other great UI’s and visualization frameworks built in Pharo.
Indeed, it is built in Pharo. However, the UI is designed as a language made of visual and interactive operators that can be programmed and combined in many ways live.
This enables a new kind of programming we call moldable development through which we construct tools for each individual development problem.
In any case, I'd be very happy to support you in playing with GT. Please ping us on Twitter (https://twitter.com/feenkcom) or via email.
These are interesting questions. Would you like to detail them?
GT is a development environment. It currently has full support for Pharo code. However, it is already possible to analyze systems written in other languages. Is this the direction of your questions?