Assets/Gravitation_Simulator.cs(23,47): error CS1061: Type `UnityEngine.GameObject' does not contain a definition for `Length' and no extension method `Length' of type `UnityEngine.GameObject' could be found.

I am trying to find the .Length of an array of GameObjects, but it says there is no definition for Length.

 using UnityEngine;
 using System.Collections;
 
 public class Gravitation_Simulator : MonoBehaviour 
 {
     public static Vector3 getRelativePosition(Transform origin, Vector3 position) {
         Vector3 distance = position - origin.position;
         Vector3 relativePosition = Vector3.zero;
         relativePosition.x = Vector3.Dot(distance, origin.right.normalized);
         relativePosition.y = Vector3.Dot(distance, origin.up.normalized);
         relativePosition.z = Vector3.Dot(distance, origin.forward.normalized);
         
         return relativePosition;
     }
 
     public GameObject Grav_Markers;
     void Start () 
     {
         Grav_Markers = GameObject.FindGameObjectsWithTag ("Grav_Marker");
     }
     void Update () 
     {
         for (int i=0;i < Grav_Markers.Length; i = i + 1)
         {
             gameObject.rigidbody.AddForce(getRelativePosition (gameObject.transform.position, Grav_Markers[i].transform.position));
         }
     }
 }