The ability to fix things on the fly and have the whole system as your ide/debugger is nice, but that's not very practical for the end user.
The primary goal of Glamorous Toolkit is not to be a technology for expressing systems in. Its purpose is to be an environment with which to work with systems written in various technologies.
The differentiator to other environments is that here you can create contextual experiences (made out of micro tools) while you develop.
That said, of course, the first language targeted is the main language in which the environment is built: Smalltalk. And you can build interesting systems with Smalltalk today.
The end user does not have to see the development tools. You can just fine create web applications.
Collaboration is handled through Git (all sources of GT are on GitHub).
You mention TDD. Moldable Development is complementary and can be quite interesting to explore :). And more importantly, you can use GT to employ Moldable Development in your system development.
The way Java IDEs work on their virtual project filesystem mimics a Smalltalk image for example.