＜スクリプト＞

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class SlowLookAt : MonoBehaviour
{
    public GameObject target;
    private float distance;

    public float speed;
    private float step;

    void Update()
    {
        // ２点間の距離を計測
        distance = Vector3.Distance(transform.position, target.transform.position);

        // 一定の範囲内に入ったら実行
        if(distance < 5f)
        {
            step = speed * Time.deltaTime;

            Vector3 targetDir = target.transform.position - transform.position;

            Vector3 moveDir = Vector3.RotateTowards(transform.forward, targetDir, step, 0f);

            transform.rotation = Quaternion.LookRotation(moveDir);
        }
    }
}