Samsung Gear S is best when used to Samsung Galaxy smartphones, but that does not mean you cannot use Gear S with any other smartphone.
Though Samsung made it clear that Gear S only supports Samsung Galaxy phones officially, but the Gear S has a hidden setting through which you can pair Gear S with any Android phone. This method also works with Apple iPhone.
When you use Gear S with any phone other than Samsung Galaxy phones, the functionality will be limited. Many features will not work properly, which includes S Voice, Nike Running, News Briefing, Milk, etc.
Steps to use Samsung Gear S with iPhone and any other Android phone
Step #1 Back-up your data on Gear S
This is the most important step. You need to back-up everything you have right now on Gear S. When you complete back up go to Step # 2.
Step #2 Factory reset Gear S
You need t factory reset Gear S now. Just go to Settings → Reset Gear.
Note after this all the data on your Samsung Gear S will be lost.
Step #3 Reboot Gear S
After doing factory reset reboot Gear S and this will give the option to pair. Press next and go to the next screen.
Here tap on the watch symbol on the top 25 to 35 times. You will see a message “Please press BT address while 3 seconds”.
After that pop-up goes away, press the watch icon for about 5 seconds. After this your Gear S is ready to be used with any phone.
Step #4 Pair Gear S with your phone
Now go to Settings → Connections → Bluetooth → Make Visible
Go to your phone Bluetooth menu and you should see Gear S. Pair the devices.
As I said before, you will miss some functionalities if you will use Gear S with non-Samsung smartphones.