How To Make A 2D GUI Arrow Point At A 3D Object

I'm assuming that you want the arrows to show only when a target is not visible and the target is in front of the camera. Make a plane with an arrow texture. The code is cleanest if the arrow points up with rotation (0,0,0) and the tip is at the anchor (0,0,0). This can be done using the build-in plane and an empty game object or using the CreatePlane editor script. Place the plane at the near clippling plane of the camera and size to taste.

Use WorldToViewportPoint() to translate a targets position to viewport coordinates. If the x and y values are between 0 and 1, the target's center point is visible. If the value of z is positive, then the targt is in front of the camera. From this you can devise a test so to show and hide the arrow. You may want to widen the parameters so the arrow does not show for targets that are nearly past your position and nearly in view. This will mitigate partially show targets having an arrow.

Below is a bit of starter code. Note I place the arrow on an ellipse touching the sides of the viewport. I leave placing the arrow on the edges of the viewport to you.

 using UnityEngine;
 using System.Collections;
 
 public class ArrowTest : MonoBehaviour {
     
     public GameObject goTarget;
 
     void Update () {
         PositionArrow();        
     }
     
     void PositionArrow()
     {
         renderer.enabled = false;
         
         Vector3 v3Pos = Camera.main.WorldToViewportPoint(goTarget.transform.position);
         
         if (v3Pos.z < Camera.main.nearClipPlane)
             return;  // Object is behind the camera
         
         if (v3Pos.x >= 0.0f && v3Pos.x <= 1.0f && v3Pos.y >= 0.0f && v3Pos.y <= 1.0f)
             return; // Object center is visible
                 
         renderer.enabled = true;
         v3Pos.x -= 0.5f;  // Translate to use center of viewport
         v3Pos.y -= 0.5f; 
         v3Pos.z = 0;      // I think I can do this rather than do a 
                           //   a full projection onto the plane
         
         float fAngle = Mathf.Atan2 (v3Pos.x, v3Pos.y);
         transform.localEulerAngles = new Vector3(0.0f, 0.0f, -fAngle * Mathf.Rad2Deg);
         
         v3Pos.x = 0.5f * Mathf.Sin (fAngle) + 0.5f;  // Place on ellipse touching 
         v3Pos.y = 0.5f * Mathf.Cos (fAngle) + 0.5f;  //   side of viewport
         v3Pos.z = Camera.main.nearClipPlane + 0.01f;  // Looking from neg to pos Z;
         transform.position = Camera.main.ViewportToWorldPoint(v3Pos);
     }
 }