There is a plethora of hand held gimbals for Smart Phones today, but just in case you were looking at the Zhiyun Z1-Smooth-C, I just happened to have one sent over and here's a video overview and test using the iPhone 6s.
The Zhiyun Z1-Smooth-C gimbal is easy to calibrate through a computer system, or without a PC. The trick is to remember the sequence of positions you need to flip through, and finding a level surface to do this. But once you're calibrated it should be fine.
I can't confirm this, but by looking at the build and how it feels, I don't believe this has any encoders. This is something all gimbals are starting to introduce, and possibly because this uses it's own software it may not be supported yet.
The Zhiyun Smooth is a pretty straight forward system to get up and running, and it's nice that it does fold completely flat for travel. It can support smartphones with screens as large as 7" inches, which includes the iPhone 6 Plus. You can find the product available (here).
The Lanparte HHG-01 3-Axis Gimbal worked perfectly to stabilize my handheld shots as I moved the camera around and kept my horizons perfectly level. I had absolutely no issues with drifting and rolling. It's certainly one of the best options available for those who want the option to shoot with both a GoPro or switch off to an iPhone instead of investing in a gimbal that can only be used with one.