How to change the redirect URL of a form that's already been set in a module?
You should be able to achieve this by adding your own submit function to the end of the submit handler array and overriding $form_state['#redirect'] there. Example code:

function mymodule_form_node_gallery_manage_images_alter(&amp;$form, &amp;$form_state) {
  $form['#submit'][] = 'mymodule_node_gallery_custom_submission';
}

function mymodule_node_gallery_custom_submission(&amp;$form, &amp;$form_state) {
  $form_state['redirect'] = 'my_url';
}