bpiper is right in that altering the default gallery.php won't do anything if you aren't using that template to display your images. Crazy concept, I know.
You're also right that you won't be able to use $image->ngg_custom_fields["Your Field Name Here"] outside of the default NGG templates. Luckily, I have an API for that. You can use the function nggcf_get_field($pid, 'Field Name').
I don't have this jQuery carousel plugin installed, so I don't know what variable the picture ID is in, but that's needs to be passed as the first argument. Then the second argument is just the field name as normal.
Let me know if you have any more problems