Still a long long way to go, as far as really being a substitute for a GM, but I've found at least two programs that seem like a step in that direction. One is an iOS app
and the other a python app available on GitHub
(the last link is an article about the Python program, which includes a link to GitHub for the source code).
Both apps use a "corpus" of text on which to base their suggestions for words that you should write next. One of them claims they have some Shakespeare as one of their seed texts, for example. These may allow you to write somewhat convincing take offs of the source material. The author of the article I listed thinks that the python program does anyway, because "[w]e've established such clear conventions and expectations for these websites, events, and TV shows that an algorithm can convincingly emulate them."
I think that this can be somewhat leveraged for emulating a GM. If you let the predictive output sort of guide you without much of an agenda other than keeping the output coherent and contextual, it feels like the computer is putting a GM response together for you semi-autonomously. This has been pretty good for me in terms of playing solo.
p.s. By the by, I've tried to put a tool together myself