`<gcse:search>` is not NNF code! (it's not even any code I've seen before)
Have you modified the templates? You can include such an element, if the "gcse" namespace is registered when the template object is created -- example from the documentation: http://camendesign.com/dom_templating#api-new The equivalent location in NNF's code is here: https://github.com/Kroc/NoNonsenseForum/blob/master/lib/functions.php#L62

(this highlights an interesting issue that NNF should provide a way for custom namespaces)