I know the history of J, and I agree with what they did. I also fully agree with your observations regarding Tk. GTK+ is a far better choice than Tk. There is one important difference though. Integrating GTK+ is a huge job! Integrating Tk is much easier. Bang for the buck, Tk is a good first pass at enabling a GUI interface of some sort.
The work done on APL's file systems and code cleanups are far more important to me. I just think that adding sockets and a GUI interface at some point would present GNU APL as a total solution. Having said all that, however, I certainly think the present course of tightening up the code, adding more standard APL facilities (trace, stop, etc.), and a file system are top priority. I am just bring up some of theses ideas.
Thanks!
Blake