If you're looking for a function that will trigger continuously without any breaks between firing, you want to use GetAxis() rather than GetButton(). GetButton() is good for buttons you want to hold down but want a slight pause between events firing (think holding down a fire button, and the gun shoots bullets with breaks in between them). GetAxis() works better for events involving movement because of this continuous frame-rate independent execution.
GetAxis() works a bit differently, as it returns a float value rather than a Boolean, such as GetButton(). It is also best suited within an Update() function. So, for example, you can check whether the horizontal movement is occurring, as follows:
void Update () { float horizontalValue= ...
