Call to undefined method App\User::gravatar() While the method exist

Please I'm new to Laravel. I kept on having issue each time I try to define a use an if statement or declare variable in the view.

Actually Im trying to display a gravatar in my Admin Dashboad. All I get is: Call to undefined method App\User::gravatar() (View: C:\wamp64\www\blog\resources\views\layouts\backend\navbar.blade.php)

<ul class="nav navbar-nav">
              <li class="dropdown user user-menu">

                <?php $currentUser = Auth::user()  ?>

                <a href="#" class="dropdown-toggle" data-toggle="dropdown">
              <img src="  " class="user-image" alt=" ">
                  <span class="hidden-xs"></span>
                </a>
                <ul class="dropdown-menu">
                  <!-- User image -->
                  <li class="user-header">
                    <img src="
                    " class="img-circle" alt="User Image">

                    <p>
                       - Web Developer
                      <small>Member since Nov. 2012</small>
                    </p>
                  </li>
                  <!-- Menu Footer-->
                  <li class="user-footer">
                    <div class="pull-left">
                      <a href="#" class="btn btn-default btn-flat">Profile</a>
                    </div>
                    <div class="pull-right">
                            <a class="btn btn-default btn-flat"
                            href=""
                            onclick="event.preventDefault();
                            document.getElementById('logout-form').submit();">Logout
                            </a>

                            <form id="logout-form" action="" method="POST" style="display: none;">
                              
                            </form>
                    </div>
                  </li>
                </ul>
              </li>
            </ul>

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