You know what it actually if for the older M-Series , BUT , I just went and tried it on my '14 PC with telescoping post and it actually fits pretty darn good!
I think a GPS would with no problem. I just tried putting the old school 35 mm Canon camera in it with the big round lens and it almost fit. about an inch short height-wise.
Interior dimensions : 7" wide at the top , 3" wide at the bottom , 5" in height.