Few days back I tried the Desktop api of Gears. Its really cool option to access the applications by selecting desktop short cuts.
You could use "createShortcut" method and when you invoke it, a dialog will be prompted for user's permission. Once the download is successful, a folder "icons#desktop" will be created. To know about the file locations, refer url.
Initially I tried using icons of size 132x27 & 18x18, but got some error message. However when I tried downloading the icons of size 16x16 and 32x32, I was successful. Not sure if there was any issue with size or the icon itself.
Refer API:
http://code.google.com/apis