R – Defining String Literals?

You can check if [string length] == 0. This will check if it's a valid but empty string (@"") as well as if it's nil, since calling length on nil will also return 0.

1. You're creating a pointer that is initialized to nil. If you're familiar with C or Java, then nil is basically the same thing as NULL or null -- it's effectively a pointer to nothing.
2. Yes. @"StarWars" is called a "string literal". Think of a string literal as a "permanent" string that is stored by the program, and gName (in this case) is a pointer to the memory location of that string.
3. Yep -- that line makes sure that the incoming gName is not the same as the object's geekName variable. Assume that check wasn't there, and gName and geekName pointed to the same string -- then the next line would release that string, which could make the next line invalid (it would be copying a released string -- very bad!).
4. Assuming geekName is still nil, then yes, nothing is released; or rather, the release message is sent to the nil object, which does nothing with the message. (Unlike C or Java, in Objective-C, you can send messages to nil -- but all such messages are ignored, and nil is the return value from all such messages).
5. You create a copy of gName in case gName is really an instance of an NSMutableString -- you don't want other code to mutate the string that you are treating as immutable. But what happens to the original object pointed to by gName? Well, it depends on what the caller does with it. It could be released, it could be retained, it could be passed to another object...you don't really know. In this particular case, since gName points to the string literal "StarWars", nothing happens to that object (since it's a string literal -- see above). And the pointer itself is only valid in the scope of this method, and goes away once the method returns.