Gyro are used in helis is to counter act the force the main rotor blade it generates. That means its used on the tail rotor to stop the heli spinning and keep the heli pointing the direction you wanted to go. If there are wind blowing on the side of the heli , the gyro will counter act the force and apply more angle to the tail balde to stop the tail pushed on its side. You can fly helis without a gyro BUT it well be VERY difficult to control without the aid of the gyro. Just think of it a car with power steering with gyro... no power steering without gyro.