Sadly, as you know; the G12 is a compact camera with a "fixed lens"; meaning, you cannot change the zoom lens that came with the camera.
Canon do made a add on conversion lens for the G's but they are mainly the telephoto variety. If you want ultra wide angle add on, you will have to go to third party manufacture or Nikon!
Of all the wide angle conversion lens ever made, Nikon's WC-E75 is, as far as I know; optically, the best!
Shot with my G12 and Sunpak RD2000 bounced.