I have a LARMOUR glass protector on mine right now, which is supposedly by GGS, but mine came dirty and had bubbles in it when installed, unlike the GGS protector for my 6D. I'll be ordering the GGS name brand one soon, maybe tonight while I'm thinking of it and let you know how that one is. I LOVE the GGS protector on my 6D. They're made of glass, super thin to the point you really can't tell they're there, yet they don't scratch easily like a plastic sticker type protector would.
My LARMOUR protector is well made and it works exactly as it should, but it has a couple of bubbles in it (have no ill-effect on the touchscreen) and the inside was dirty when I got it (can't clean it, but again, no negative effect on the touch screen). I would've sent it back, but I had an airsoft game I was dying to bring the M to, so I just left it on.