Question

Menu Callback

function content_form_select($id, $sid){
    $type = check_content_type($sid);
    if($type == 'video')
        // Render content edit form
        return drupal_get_form('content_video_form', $id, $sid);
    else if($type == 'gallery')
        // Render content edit form
        return drupal_get_form('content_gallery_form', $id, $sid);
}

Video Form Generator

function content_video_form($id=null, $sid=null){
    return array('#value' => 'Video form is getting rendered.');
}

Gallery Form Generator

function content_gallery_form($id=null, $sid=null){
    return array('#value' => 'Gallery form is getting rendered.');
}

It does not render form this way

Answer 1

The drupal_get_form expects to receive a $form array, which then contains the form elements. Using one of your example functions above, the following change works for me:

function content_gallery_form($id=null, $sid=null){
  $form['example'] = array('#value' => 'Gallery form is getting rendered.');
  return $form;
}