JK2Radiant wasn't derived from GTKRadiant.. they're two separate editors.
btw, you can change the way GTKRadiant looks. It's not exactly the easiest thing to do and I kinda forgotten how to do it but it involves editing text files and all that. Personally, I couldn't care less that it looks less attractive than JK2R, as long as it works better. I think it has more functionality and ease of use.
I do recommend that beginner mappers stick to JK2Radiant and then move on to GTK after a while though.