How to display the content HTML of a page without displaying the gallery code as well

I may be misinterpreting you, but I think what you want to do is run through strip_shortcodes

  $page_data['content'] = strip_shortcodes($page->post_content);
  $page_data['content'] = apply_filters('the_content', $page_data['content']);

Alternately you could conditionally hook that function to the_content as demonstrated on the Codex Page.