Gyroscope rotation offset

Hi! I'm making a game using the gyroscope (2d game). When I rotate my phone, it rotates the player's character. However, when my phone is totally straight (landscape), my character isn't straight. (see screenshot) I tried to see and there's like an offset of 70° (when my phone is at 90°, my character is at 160°)

Here is my script:

     void Update()
     {
         transform.rotation = GyroToUnity(Input.gyro.attitude)
     }
     
     Quaternion GyroToUnity(Quaternion q)
     {
         return new Quaternion(0f, 0f, -q.z, -q.w);
     }
 

If I return a new Quaternion, it's just because I only want the Z rotation (because it's a 2d game).

I tried to do a simple addition like -q.z - 70 but then it does something weird and only moves between 179° and 180°. If anyone has the answer ^^

Thanks in advance!