Anything with a rough surface should provide some grippy goodness, as well as some smooth TPU and PC thin cases that will work as well. Adding to the manufacturers already mentioned, there's Ringke and OtterBox for thin or thick cases. I'm not keen on the G6 OtterBox designs (why leave that much of the back exposed?) or the colors however. I have a Ringke Fusion Air which is smooth but has a nice tacky grip, as well a dbrand Black Dragon skin which is really rough - I originally bought it to protect from scratches without using a case but the discovery that it feels like sandpaper was a bonus.