Hallo BeckH,
Not having a G11 I can only guess at what these ctg files are although I believe it to be a good guess. When shooting jpeg images the camera also produces thumbnail files which allow the files to be viewed in Canons own software. On my old G5 if memory serves me correctly these files had the suffix thm. If you check, you'll probably find that for every image you will have an associated ctg file. Most editing applications i.e. Elements, Photoshop and others, produce their own thumbnail images so these ctg files aren't really necessary.
Edit: out of interest I did a quick Google and came up with this site. Read the second post by Chuck Westfall who is one of Canons own gurus.
http://www.prophotohome.com/forum/ca...-ctg-file.html
It's basically as I said earlier though, these files may be required if you only use Canons own software for editing etc, but if you are using most other image editing products then these produce their own thumbnail images so the ctg files are not important. If you want to convince yourself, then drag and drop one image plus it's associated ctg file into a temporary folder followed by deleting the ctg file and sending it to the recycle bin. Open the image in your editing programme - if it opens ok then you'll have confirmed that you can safely ditch these ctg files and save hard drive space.