Just got back from a 65 mile trip in the Sierra. First time out for me with a 2012 Gorilla. My go to pack for the last few years has been a Mariposa Plus. This is the second trip on this pack as my sons girlfriend used it on the High Sierra Trail a few weeks ago. I did not hear any concerns from her, but, I did not specifically ask about slippage.
As for me, I had slipping on the shoulder straps. I would notice the pack pulling away a bit more from my body and could feel the sewn webbing had slipped all the way to the buckle. Fortunately, the webbing does not have a lot of "memory" to hold it in place so I just leaned to one side to take pressure off the webbing and would cinch it down from one side to the other.
I do have to say I really like the 2012. The top pocket held my drivers license, keys and my permit. I never had to open it again and was glad to have those items secure. This pack wears real well and I am real pleased with it.