Currently it's not possible to obtain this item.
If you check the description of this item in the Cyclopedia it says
"This traditional Orcsoberfest gamsbart shows a bit of use but is still in good shape and will clothe any sophisticated ogre just fine"
From that we are able to conclude that it's related to the Orcsoberfest.
Orcsoberfest is celebrated twice a year starting at the second Friday of March and October, lasting for a week each time. Since it's not possible to obtain I can think of two ways that it would be possible to obtain it:
The first celebration of Orcsoberfest was last October and the first time you defeated the final boss, King Chuck
you recieved a pair of Lederhosen and they have a similar description to the Traditional Gamsbart Hat. It is possible that it's a reward for another Orcsoberfest.
Lederhosen
"Made from dried and tanned orc skin, these tranditional Orcsoberfest leather trousers are sturdy and fit a wide range of orgres."
Except the Traditional Gamsbart Hat and Lederhosen another 3 items with similar names and descriptions were added. It may be that they will be rewards instead.
Traditional Neckerchief
"A traditional neckerchief which complements every traditional Orcsoberfest look."
Traditional Shirt
"A traditional white Orcsoberfest shirt. Not worm by many ogres during the actual festivities anymore, however."
Traditional Leather Shoes
"Soled with sturdy orc hides, these traditional Orcsoberfest leather shoes are robust and useful."
Another theory is that it's obtainable by trading it for Festive Points with Xaver at the Orcsoberfest island.