At first glance a couple things stand out to me:
- gform_confirmation is a filter, not an action. You’d use add_filter(), not add_action()
- Your parameters in the callback function are a little crossed up. As the filter doc notes the entry object is the third parameter available with the filter, but you’re trying to get the entry from the second in your code. With how your function is declared $form would be the entry object and $entry would be the form object in the context of your function.