The SB-25 should be safe to use, as its sync voltage is around 4V. The Powershot Gs were initially limited to a 6V voltage limit, and I'm not sure if they were upgraded to the 250V limit the post-300D dSLRs are.

It should fire in sync on the hotshoe of the Powershot G9 (my SB-26 fires on my G9), but you may want to tape over the non-sync pins (those four arranged in a little square below the main sync pin) to avoid cross talk with the hotshoe contacts. The pins shouldn't hit the contacts (Nikon places their pins differently), but the clearance isn't very big on the lower two pins.

However. Firing in sync is all that's going to work. You have to set the power on the flash in M mode and manually set the zoom. You can use the A mode to automatically set the flash power, but you'll have to set the ISO and aperture you're using on the flash as well, since it can't talk to the camera.

Unlike the SB-26, however, the SB-25 does NOT have a built-in optical slave, so to use it off-camera, you'd have to use a TTL cable (the Powershot G9 has no PC sync port, only the hotshoe) or radio triggers. I would, however, recommend going the radio trigger route. A full-sized speedlight with 4xAA batteries in it is so much bigger/heavier than a G9 that it's an unwieldy combination, and the stress on the flash foot is not ideal. Radio triggers being much smaller/lighter, it's an easier combination to handle.