You can use the map and GPS functions of your smartphone in Turkey. You can get general positioning just via Wifi (if you're near Wifi hotspots, as in a city), but for the most accurate location readings you will need to have access to mobile (cellular) data service. The cheapest way to have data access is to buy a Turkish SIM card for your smartphone. More...
I've used the mapping and driving directions functions of my Apple iPhone 6 and a Turkish SIM from Türkcell with good results, though the spoken directions mangle the Turkish pronunciation.
Without a Turkish SIM, data roaming using your home-country telecom company and a Turkish telecom can be quite expensive. More...
You can use your Garmin, TomTom or other Global Positioning
System (GPS) device in
Turkey, but maps of Turkey may not
be included in the map set that you
already have on your device.
For Garmin GPS devices,
you'll probably want the City
Navigator Europe NT — Turkey map set. Here's
what it covers.
For TomTom GPS
devices, you can buy
and download Turkey maps here.
GPS Traveler Tips
Here are tips for traveling with a
GPS system in Turkey from TTP traveler
Andrea Hein (edited by Tom Brosnahan):
—Be sure to get
an English version of the GPS maps,
but when programing your destination,
you first have to put the province [il in
Turkish] before you can input the town.
For instance, I wanted to put in Şirince,
but it wasn't recognized until I first
input İzmir, which is the province.
—Tricky to understand
some of the Turkish that's
written in the locations on the GPS....for
example when putting in İzmir
it says the Turkish word for airport [havalimanı] and
if you didn't know that the word meant
airport, you would be confused.