Hallo Peter,
you didn't mention your OS. In case you're Mac user, there are GUI
controls ("MAUI elments") in MiniAudicle for OS X -
http://audicle.cs.princeton.edu/mini/mac/
I can't say more about this topic because I'm on Linux.
My personal choice for ChucK GUI controls is Processing with ControlP5
and OscP5 libraries.
Antanas