Ruby and Arduino over Serial
Firstly, let's look into documentation for Ruby IO#gets:


  Reads the next "line" from the I/O stream; lines are separated by sep ...


Secondly, look into documentation for Arduino Serial.print:


  Prints data to the serial port as human-readable ASCII text ...


Finally, you don't print new line char \n in your "arduino sketch" above.

I recommend you to use Serial.println in the last line from your sketch:

void setup() {
  Serial.begin(9600);
  // ... do some gyro stuff here
}

void loop() {
  gyro.read();

  Serial.print((int)gyro.g.x);
  Serial.print(" ");
  Serial.print((int)gyro.g.y);
  Serial.print(" ");
  Serial.println((int)gyro.g.z);

  delay(500);
}