It will be just the same as any sugarfree lowfat yoghurt, whichever GL book you read will give you guidelines about this type of food under that plan. It's a protein food but also has some carbs so will have a GL value. Yoghurt in moderation is encouraged by many of the plans and can make a nice pudding with berries and seeds.
I personally really like the lowfat Greek and the lowfat fruit flavours (which you add your own sweetener to) as I found the skimmers to be quite bland and watery. But some people really like it!