The intent of PhoneGap is not to wrap an existing website. Although this can be done technically, your app will also get rejected by Apple (and potentially other stores). Google isn't typically as picky, but even so, it's still a bad idea, because you need local code to handle the case of no (or a poor) network connection.
You can create an app that interfaces with your existing website if you want, but you'll need to learn how to create single-page applications using JS, CSS, HTML (you'll probably want to investigate a framework like Ionic or Framework7), and XHR (XMLHttpRequest) for communication to your website. Even then, Apple may reject if they feel the result could live as a web app -- you'll need to use some device functionality to get around that (persistent local storage, SQLite, sharing, etc.).