I have used the GGS protectors in my Rebel, 7D, and 6D. In all cases I was able to carefully apply the back screen protector without leaving any bubbles. When I sold my Rebel and 7D, I was able to remove those protectors just fine, no glue residue, and a spotless, like-new screen.
The top screen protector wasn't the same experience. That one has an adhesive on the black frame and it's slightly flexible, this left a small air bubble no matter what on my 7D and 6D, as air would always be trapped due to the protector's design. The bubble didn't bother me as I managed to make it as small as I could. Removal of that protector left a glue residue that I was able to easily remove before selling my 7D.
Now it was never my experience that there was anything on the 6D design that makes things difficult on this front, it was rather about the limitations of the protectors. IMHO, do what you think it's best, stop trying to find faults on the 6D, and enjoy it gorgeous IQ. It still brings smiles and a warm feeling to my heart.