This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I believe that the class must already exist -- at least for libraries such as Gson (I think Jackson is similar, though I haven't used it much). You can get Gson to create a Map structure that represents the JSON, but it won't magically create a new class definition and load it.
If it's a known construct, why not create a class to represent it? Or is it completely random?