The Ghastly Gibus was introduced to the game on October 29th 2009 as part of the Haunted Halloween Special update.
It was first unlockable by visiting a hidden page linked from the Halloween 2009 update information and 10,319 players obtained it through this method. Interestingly the Ghastly Gibus was the second item to be unlockable by visiting a hidden web page, the first being the Gentle Manne's Service Medal. After the 10,000 initial Ghastly Gibuses were given away, the only way to obtain this hat is by completing the Ghastly Gibus Grab achievement: Dominate a player wearing the hat.T
The Ghost of Zepheniah Mann on the Halloween event version of the map Harvest can also be seen wearing this hat and the initials "ZM" are embroidered on the inside. The hat also bears some resemblance to the Demoman's Scotsman's Stove Pipe.
On the Halloween 2010 Scream Fortress update, the Ghastlier Gibus style was made available, adding a jack-o-lantern medallion tucked into the brim, referencing the Horseless Headless Horsemann.
On the Halloween 2011 Very Scary Halloween Special update, the Ghastlierest Gibus style was added which features an eyeball peaking out of the top of the hat; a reference to Monoculus.