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.
List l2 is a list of anything. You can assign it any type class implementing List. Moreover, this list is not generic (it has not the List<...> notation). So it can also be assigned to any kind of generic list. It's not affecting it in any particular way. It's a good old non-generic list which was used before Java5.
You'll get a compiler warning about that line because it is dangerous. Your example has already shown that you now have an Integer in your List<Gum>. When you now try to iterate over it you will get a ClassCastException.