No i have not tried these on garters yet but a friend of mine uses this on wildcaught mangrove snakes and has great results and he said it increases their appetite. That worm guard is all natural and works as a laxative so it may help with impaction too. It is cheap and could be sprinkled on some fish and then fed to the snakes.
I checked out the worm guard... it's a laxative that helps the worms get passed.... doesn't kill them. I have some, but haven't used it (have been using Panacur for deworming) May have to give it a try since I have it (was given to me by a friend)