How to use a third-party form in application page? (Forms like Contact form 7, Gravity forms and even Google Forms)

The Pro version comes with custom forms integration. You can find the option to choose third-party forms in the job edit screen. 

You also have the option to replace the default form with a button to an external URL or even disable the form completely. 

Please note that, if you use a custom form the applicants' data will not be accessible for the plugin. Applications will be handled by the form plugin you are using.