LottoClient class is not in the same package as GuiClient class that's why you can't see setGui() method, which has a default access modifier.
If you want to expose setGui() method to all classes mark it as public otherwise put LottoClient in package abc.
The follow is the link you need to understand the access modifiers
http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html