Install as system app is grayed.

It is disabled since it is not working properly in all cases...
It will be enabled if you enabled beta features and the experience level is set to power user.
Or move cpu tuner from from /data/app to /system/app by hand.
(You will have to mount the system partition rw, i.e. run `mount -o remount,rw /system`.)
If installed as system app cpu tuner can switch GPS on and off.