How so? I get that going the route of Python and Lua and using a general purpose data structure for the environment isn't optimal. But g-fu exposes the already existing environment as a separate type, the only thing it knows how to do is map symbols to values and access is restricted to a few special forms just like any regular Lisp enviroment.

Well, context oriented programming is all about separating the system into layers of behavior that may be flexibly and conveniently composed to get the right mix of functionality. Which is exactly what g-fu is doing with first class environments.