Both projects are free implementation of Java standard libraries. Libgcj's home page says it was merged with Classpath, But they still have different packages in Ubuntu.
So what's the difference? Is one better than the other? What depends on each other?
The GNU class text is a replica of the Sun's built-in Java classes. Libgcj is a basic library that runs compiled Java application for native code via GCJ. They are under one umbrella, but there are different objectives.