Creating an onSaveEntry event in Guest Entries Plugin
It probably would be better to just write your own plugin to do this, so that you don't have to worry about patching in your changes every time the guestEntries plugin is updated — and it could technically be just one file, although the recommended craft method would be to create a service method to handle the events.

In your plugins main class file, just create an init method that listens for the entries.saveEntry event for the section in question:

public function init()
{
    parent::init();

    craft()-&gt;on('entries.saveEntry', function(Event $event) 
    {
        $entry = $event-&gt;params['entry'];
        if ($entry-&gt;section == 'MySection') 
        {
            if ($event-&gt;params['isNewEntry']) 
            {
                craft()-&gt;myplugin-&gt;handleNewSectionEntry($entry);
                // or do something directly
            } 
            else 
            {   
                craft()-&gt;myplugin-&gt;handleUpdateSectionEntry($entry);
                // or do something directly
            }
        }
    }); // edited
}


If you need to test whether it is a 'guest' entry from the front-end vs the CP you can add:

if (!$currentUser) { ... }