Samsung's Gear S2, Gear Fit 2, and Gear S3 smartwatches finally work with the Apple iPhone. Samsung said in December 2015 that it would bring iOS support to the Gear S2 by the middle of January 2016. It took much longer for full support to actually roll out.
To use one of the wearables with an iOS device, you must download the Gear S app from the App Store. Setup basically consists of tapping the "connect to Gear" button that appears in the app.
You must be using an iPhone 5 or later that's running at least iOS 9.