It is my understanding that HubPage is designed for original content. So, if you are simply thinking of re-posting works that are part of the public domain from Project Gutenberg on HubPages as a shortcut to posting more Hubs I would advise against it. Even if it is acceptable by the user guidelines and rules, it waters down the usefulness of the site for people who seek good content here and it floods HP with what is essentially Internet garbage drowning out HP users who are developing new material.
However, if you are interested in quoting Project Gutenberg materials in a Hub as a reference source to make a case or an argument, then go for it. Even though the works are in the Public Domain, you should quote your source as a courtesy.