Guk was merged with 2 other servers into the Halls of Fate server. As an aside, you can easily see which server your characters are on by looking at your characters on the character select screen. Any given character's server is indicated directly under the toon's name.
When you next log in, don't choose to log directly into a character, instead go to the char select screen. This will refresh you character lists and will show all of your characters and their server(s).
