turning on and off gravity help

am doing my first game where my player has to move in x and y directions according to the inputs given from keyboards and has to fall down if there is no inputs are give (i.e) if no keys are pressed.

is there a way to turn on gravity when there are no inputs and turn off if any keys are pressed.

previously i was doing gravity manually. the i used previously is below

 using UnityEngine;
 using System.Collections;
 
 public class player_script_2 : MonoBehaviour {
     
     Vector3 move;
     float accleration = 10;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         
        move.y = Input.GetAxis ("Vertical");
        move.x = Input.GetAxis("Horizontal");
  
        //Check if no buttons are pressed
        if(Input.GetAxis("Vertical") == 0f && Input.GetAxis("Horizontal") == 0f) {
          if(transform.position.y >= -4.45f) {
           move.y -= 2f;   
          }
        }
         
         print ("before "+ move);
         move.Normalize();
         print ("after "+ move);
  
         float xdir = move.x * accleration * Time.deltaTime;
         float ydir = move.y * accleration * Time.deltaTime;
  
         transform.position = new Vector3(transform.position.x + xdir, transform.position.y + ydir, transform.position.z);
     }
 }

sadly according to my script, the player's jumping or standing on a higher level wont happen as i have given a check for y position. need help...