So I activated the plugin and tried to load the Customizer.
I received this error from inside one of wpDiscuz’s files:
Uncaught TypeError: Cannot read property 'trim' of undefined
I then activated TwentySixteen and it worked.
Then I re-activated GeneratePress and the Customizer is working.
Very strange, but I do believe it’s something in the plugin.
Can you try re-activated GP and seeing if the Customizer loads after having TwentySixteen activated?