Nope, even if Realbot is under GPL the Realbot code IS (C) Copyright Stefan Hendriks (maybe Christian Authmann as well) and protected under appliable copyright law. So you had better not remove that Copyright notice.
Why you have to release your source code is: You have used code which is (C) Copyright Will Day and you have to obey his license.
What has something wrong is: you are restricting the redistribution as that way. Actually that's also NOT "violating" GPL, but by applying the GPL that clause just turned into nothing. I thought you don't want your software to be redistributed so I pointed that out. (and if you do want others redistribute the software perhaps you should remove that instead of the copyright thing
)
(btw, has Christian Authmann "offically widdrawn (withdrawn)" now ?)