IPA file from here to your computer. Once you have downloaded the file, connect your iPhone to your computer and double click on the IPA file and tap install. Once the synchronisation is done, navigate to the Settings > General > Device Management , then trust the Samsung certificate. Then do a factory reset of the Gear S2 or S3, and pair it using the Gear Manager app via Bluetooth.
It should be noted that there is only a small number of watch faces available for iPhone users which is kind of expected considering that the support for iOS devices isn't official yet.