I was looking for GPS solutions for my D750. Of course, Nikon has the GP-1A which is pricey and looks like a cumbersome add-on, much like the other third-party add-on GPS devices for DSLRs which uses your hot shoe as a mount. The drain on battery life is another factor to consiider, which is probably why the only Nikon DSLR with built-in GPS is the D5300.
I discovered a cheap alternative that uses the iPhone GPS. There is an app called GPS Stone GPX Trip tracking & recorder, which is available for a nominal fee. It creates a GPX file that you can email to yourself. If you have Lightroom, you can use the Map function to download the GPX file and Lightroom will associate the GPS data to your photos. The association of GPS data and the images is based on the time stamp from the image when it was taken.
This is what you do.
1. Ensure that the time on your phone is synchronized with the time on your camera.
2. Before you start your photo session for the day, turn on your app and begin recording GPS data. The app will record even if you close the app or when your phone is on standby.
3. Go ahead a shoot photos as you normally would.
4. When the day of photo shooting is over, go into the GPS Stone app and stop recording. A GPX data file will be created which you will email to yourself.
5. Download your images to Lightroom and use the Map function to download the GPX file. Lightroom will add the GPS data to the photos you took during the day.
Granted, you don't get instantaneous GPS data in you image files if you used a GPS add-on but it is cheap, doesn't drain your camera battery and less cumbersome equipment wise, at least in my humble opinion.
Ed.