Deactivate Key.

 private bool disableG; // this will be set to true or false
 private void Start()
 {
     disableG = false;// we start it at false
 }
 private void Update () {
     if (Input.anyKeyDown) // if you press down any key
     {
         if (!Input.GetKeyDown(KeyCode.G)) //and that key isn't g
         {
             disableG = true; //disable g is set to true
         }
     }
     if (!Input.anyKey) //if you're not pressing anything
     {
         disableG = false; //disable g is set to true, this will ensure if you press and release a button you'll be able to press g again even if it's in a single fram
     }
     if (Input.GetKeyDown(KeyCode.G) && disableG == false) //now we see if you're pressing g and it hasn't been disabled by pressing another button
     {
         //do stuff
     }
 }

As simple as:

 if (!Input.GetKey(KeyCode.G))
  {
          //do nothing or return
 }

,As simple as: if (!Input.GetKey(KeyCode.G)) { //do nothing or return }