If you look at the PhoneGap InAppBrowser documentation here, you can set the target of the target in which to load the URL. Set the target to _system. So it should be something like this:
window.open('https://www.google.com/', '_system');
Stackoverflow is your friend: https://stackoverflow.com/questions/20011753/phonegap-open-external-link-in-default-browser-outside-the-app
Regards,
Ronald