What to do when GT hangs?
This typically happens if you run some code that has an infinite recursion.
To interrupt the GT Processor, just type Cmd+. (MacOS) or Ctrl+. (Windows).
This should interrupt the currently running process and open a debugger.
NB: You may have to do this multiple times to get the attention of the Processor.
If all fails, you may have to force-quit GT.