Why I need to put my phone to Gear VR to debug google cardboard app?

I am using Unity3d and when I press Export & Run after the loading my phone asking me to connect my phone to the Gear VR.

The app is only for Google Cardboard!

I suppose any misconfiguration about the target platform in your Unity project. If you’re building for Google Cardboard, the target platform must be Android. And I believe you would have used the Cardboard SDK.

Still if you encounter the problem, feel free to comment me wrong.

Either you enabled VR Supported option in Player Settings or you have a oculus signature file in the PlugIns / Android / Assets folder or even both.

I recommand not testing cardboard apps on Gear VR because they are two different systems which are not intended to work together. If you want to run your game on a Gear VR drop the cardboard parts.
If you want to have your game supporting cardboard disable VR supported in the Player settings and drop the oculus parts.

In short: not mixing sdks and technology.