To clarify the manual and motor timing:
The manual recomends running the physical timing on the motor itself (if adjustable) at zero.
In truth - REGARDLESS of what BL motor you're running, the GT ESC alone controls timing advance, because it's switching into sensorless mode almost immediately after the motor starts turning ("Hybrid Drive"). Therefore - any timing advance positions on the motor (which are sensor-based) aren't being used anymore.
Obviously a BIG tuning advantage if your motor itself has no adjustable advance.