GPS icon in settings use regedit, go to HKEY_LOCAL_MASCHINE\ControlPanel\GPS Settings ,

1.Add a DWORD called "Group" and give it the value of "2" (dec)
2,keep the Hide key but change the value to 0 ,
3,Soft reset

you will see the icon in settings

By default, D810 doesn't has GPS service enabled, but it has GPS chip set, to enable the GPS just need upgrade the firmware, here is :

http://www.dopodasia.com/download/RUU_Trinity_DOPODASIA_WWE_1.23.707.6_6275_1.35.00.11_108_Ship.zip

it's very easy in steps.