I have a Zumo 660 (latest firmware & languages) and have just tried installing GLG using the method posted by Duck42.
All went well with the install up to the point where I was instructed to remove the SD card and reboot.
After rebooting, I still have no icon in tools page 2 to start the GLG, I noticed on one of the other posts a comment stating it will not work if you didn't have an icon to start with.
Yet on other Zumo 660's I have seen the icon, has anyone else got a Zumo 660 with it working ?