Don't show the upload avatar modal is allow_uploaded_avatars is false.

app/assets/javascripts/discourse/controllers/preferences_controller.js

@@ -7,6 +7,10 @@
   @module Discourse
 **/
 Discourse.PreferencesController = Discourse.ObjectController.extend({
+  allowAvatarUpload: function() {
+    return Discourse.SiteSetting.allow_uploaded_avatars;
+  }.property(),
+
   // By default we haven't saved anything
   saved: false,
 

app/assets/javascripts/discourse/templates/user/preferences.js.handlebars

@@ -49,7 +49,11 @@
       <label class="control-label">{{i18n user.avatar.title}}</label>
       <div class="controls">
         {{boundAvatar model imageSize="large"}}
-        <button {{action showAvatarSelector}} class="btn pad-left"><i class="icon-pencil"></i></button>
+        {{#if allowAvatarUpload}}
+          <button {{action showAvatarSelector}} class="btn pad-left"><i class="icon-pencil"></i></button>
+        {{else}}
+          <a href="//gravatar.com/emails" target="_blank" title="{{i18n user.change_avatar.gravatar_title}}" class="btn"><i class="icon-pencil"></i></a>
+        {{/if}}
       </div>
     </div>