When you add a new user into a group in AD, it will add that user into the counter part of that group in Google. But, when you move that user out of the group in AD, the google account will not be removed from the google groups. What google only does is stop allowing the user to post to that group.
After looking for the solution for a while, I came up with one hack that works:
1. Search for all users in AD groups which you want to sync to Google:
2. Search for all users in Google groups:
3. Compare two lists and delete Google Group members that does not appears in AD's counter part.
Here is a working script:
$ python google_groups_ad_sync.py [dry_run]
You can set up a scheduled task that runs this script after GADS.