gyro.ino

namespace {
  const int AVERAGE_NUM = 10;
  const int BASE_X      = 530;
  const int BASE_Y      = 519;
  const int BASE_Z      = 545;
}
  
void setup()
{
  Serial.begin(9600) ;
}

void loop()
{
  int x = 0, y = 0, z = 0;
  for (int i = 0; i < AVERAGE_NUM; ++i) {
    x += analogRead(0);
    y += analogRead(1);
    z += analogRead(2);
  }
  x /= AVERAGE_NUM;
  y /= AVERAGE_NUM;
  z /= AVERAGE_NUM;
  
  const int angleX = atan2(x - BASE_X, z - BASE_Z) / PI * 180;
  const int angleY = atan2(y - BASE_Y, z - BASE_Z) / PI * 180; 

  Serial.print(angleX);
  Serial.print("\t");
  Serial.print(angleY);
  Serial.println("");
  
  delay(16);
}

RotateBySerialMessage.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class RotateBySerialMessage : MonoBehaviour 
{
	private List<Vector3> angles = new List<Vector3>();

	public int averageNum = 10;
	public Vector3 angle {
		get { 
			if (angles.Count > 0) {
				var averageAngle = Vector3.zero;
				angles.ForEach(angle => { averageAngle += angle; });
				averageAngle /= angles.Count;
				return averageAngle; 
			} else {
				return Vector3.zero;
			}
		}
	}

	void OnSerialMessage(Vector3 angle) 
	{
		angles.Add(angle);
		if (angles.Count > averageNum) {
			angles.RemoveAt(0);
		}
	}

	void Update()
	{
		transform.rotation = Quaternion.Euler(angle);
	}
}

SerialHandler.cs

using UnityEngine;
using System.Collections;
using System.IO.Ports;

public class SerialHandler : MonoBehaviour 
{
	public string portName = "/dev/tty.usbmodem1421";
	public int baudRate    = 9600;

	public GameObject target;
	private SerialPort serialPort_;

	void Start() 
	{
		Open();
	}

	void OnDestroy()
	{
		Close();
	}

	void Open()
	{
		serialPort_ = new SerialPort(portName, baudRate, Parity.None, 8, StopBits.One);
		serialPort_.Open();
		StartCoroutine( Read() );
	}

	void Close()
	{
		if (serialPort_ != null && serialPort_.IsOpen) {
			serialPort_.Close();
		}
	}

	public IEnumerator Read()
	{
		while (serialPort_ != null && serialPort_.IsOpen) {
			string message = "";
			try {
				message = serialPort_.ReadLine().
					Split(new string[]{"\n"}, System.StringSplitOptions.None)[0];
				serialPort_.ReadChar();
			} catch (System.Exception e) {
				Debug.LogError(e.Message);
			}
			if (message != "") {
				var data = message.Split(new string[]{"\t"}, System.StringSplitOptions.None);
				OnMessage(data);
			}
			yield return new WaitForEndOfFrame();
		}
		yield break;
	}

	void OnMessage(string[] data)
	{
		if (data.Length < 2) return;
		try {
			var angleX = float.Parse(data[0]);
			var angleY = float.Parse(data[1]);
			var angle = new Vector3(angleX, 0, angleY);
			if (target) {
				target.SendMessage("OnSerialMessage", angle, SendMessageOptions.DontRequireReceiver);
			}
		} catch (System.Exception e) {
			Debug.LogWarning(e.Message);
		}
	}
}