<div class="container">
    {# this as teaser #}
    {{ content.field_gallery }}

    {# this as default or whatever #}
    {{ content.field_gallery }}
</div>

$variables['custom_display_mode'] = $variables['node']->get('field_gallery')
    ->view(array(
       'label' => 'hidden',
       'type' => 'teaser'
));