I can't see debug under org->gtk->settings or I have to install to get that?
You will need to install the GTK developer tools first.
sudo apt-get install libgtk-3-dev
Then you will be able to enable the inspector either via the dconf editor, or by executing the terminal command
gsettings set org.gtk.Settings.Debug enable-inspector-keybinding true
You can then launch the Inspector with Ctrl+Shift+i
In my experience, the GTK Inspector takes a very long time to load the first time you launch it after launching your GTK app. But after the initial launch, you can close it and it will relaunch much faster.