Using Custom Fields in post, need to put into the_content

I think that I would hijack the gallery shortcode.

function my_gallery_shortcode($atts) {
  global $post;
  $str = gallery_shortcode($atts);
  $project_title = get_post_meta($post->ID, "Project_Title", true);
  $project_date = get_post_meta($post->ID, "Project_Date", true);
  $project_location = get_post_meta($post->ID, "Project_Location", true);
  $str .= '<div class="project_details">';
    $str .= '<div class="project_details_title">'.$project_title.'</div>';
    $str .= '<div class="project_details_date">'.$project_date.'</div>';
    $str .= '<div class="project_details_location">'.$project_location.'</div>';
  $str .= '</div>';
  return $str;
}
add_shortcode('gallery','my_gallery_shortcode');

That should work but will print those divs whether the meta values are set or not so I’d write some conditions to keep things neat– ie. if (!empty($project_title)) {, etc.– so that you are only printing what you need to be.

You could also just register it with add_shortcode('my_project_gallery','my_gallery_shortcode'); and not take over the default gallery at all.