Both are listed as "Your Call" on the
Strobe Trigger Voltages page. That means that they might be a bit high-voltage for Canon cameras.
The flash is triggered by the camera making a short-circuit between the side & centre pin. The flashgun might simply have the two ends of its charge capacitor on those pins, in which case there's some hundreds of volts. Most have some circuitry in the flashgun which presents a much lower voltage on the pins, but still apparently over spec for modern Canons. Can't see that manual mode on the camera will protect it any better.
Not sure how much use a manual gun on the G9 would be.
Chris