GNS3 GUI repository.
If you want to update the interface, modify the .ui files using QT tools. And:
cd scripts
python build_pyqt.py
If you want to see the full logs in the internal shell you can type:
debug 2
Or start the app with --debug flag.
Due to the fact PyQT intercept you can use a web debugger for inspecting stuff: https://github.com/Kozea/wdb