If you are looking for a decent handheld 3 axis gimbal stabilizer that won't break the bank check out the Zhiyun Smooth-Q 3-Axis Handheld Gimbal Stabilizer for $139 USD on amazon. After I got a GoPro hero 5 black I quickly realized that the camera did not fit my existing stabilizer (ughhh). So while I was looking I found the Smooth-Q that could also work with a smart phone too.
I am do not record a lot of video using a smart phone but this gimbal does an outstanding job with my smart phone. I am using the FILMIC app with the phone and the stabilizer and I really like it a lot.... It also works great with the GoPro too. It uses a spring loaded clamp system so I would use caution if you are wanting to take it places that there is a lot of jolts in the recording. I would suggest also using a rubber band just so the GoPro does not pop out on a huge bump.
https://www.amazon.com/Zhiyun-Smooth-Q- ... 4HZTSVPRLX
Sidd