I just put some GC3s(I think they are similar to your GP5s a newer model) on my DS 8.3 and got the small size. I made that choice as I have small hands and will wear fingerless gloves a lot. I had read that you'd be better off being able to wrap your hands around the grips being less tiring to hold on which the smaller size supports. I rode just a short bit yesterday with the new Ergons and they feel great and the install was very easy. I used compressed air to get the old friction fit Bonragers off and it took about a minute with no damage to the old grips. Make sure you have enough bar exposed to get the new grip all the way on and then they will tighten up just fine. I'd heard of a few folks on this forum who couldn't get theirs to tighten and it must have been that they didn't get the new grips far enough on.