There are many ways to do flash photography with the Canon G9. I've given some examples here, here and here and will be giving more. As always, I recommend that you do as I do and follow Strobist for learning off-camera flash. Meanwhile, seeing the variation in the G9 flash display might help clear up some confusion.
With the G9 in Program mode, turn the built-in flash on, press the Function Set button and scroll down to the flash adjustments. The G9 display should look like this:
The idea here is that you can increase or decrease the flash effect as compared to what the G9 thinks it should have been. This adjustment is independent of the shutter speed or aperture selected by Program mode.
But if the G9 is in Manual exposure mode, the flash display looks like this:
In G9 Manual exposure mode, YOU control the amount of flash. It’s not difficult, take a trial shot and adjust the setting with the Control Dial. Unfortunately, the adjustments are limited to Full, 2/3 Full and 1/3 Full power.
With external flash mounted on the G9, the display looks different. Here’s the display with a Canon 580EX mounted in the hotshoe; again, the G9 is in Manual exposure mode:
With the 580EX mounted and the G9 in Manual exposure, you still must manually adjust the flash; however, you can turn down the flash to 1/64 power (and to 1/128 with the 580EX II). The display is the same whether the 580EX is in ETTL or its own manual mode. It seems easier to me to leave the 580EX in ETTL and make the adjustment on the G9. A nice touch is that even older Canon flashes like the 420EX and 380EX that do not have manual adjustments on the flash can be adjusted with the G9 just like the 580EX.
If a non-Canon flash is mounted, the G9 display looks as shown below:
The flash adjustments are grayed out and are not usable. You’ll have to adjust the non-Canon flash through its own on-flash adjustment (if any). This isn’t particularly difficult; I do it frequently with a Nikon SB-28 and SB-24.
And yes, that’s a door frame in my illustration photos! All photos of the G9 taken with the old faithful G3.