The Gundabad Orcs are the orcish tribes who took into possession of the ancient dwarven city of Mount Gundabad. These Mountain Orcs from the north were led by independent Chieftains, not directly ruled by Mordor.
Some time during the Second Age, the orcs that had survived the destruction of Angband were forced to wander the Northern Waste and find a new home. This was when they attacked the mountain and took it from the Dwarves. In the Third Age, the Mountain Orcs of Angmar officially stated Gundabad as their capital which angered the Dwarves greatly. Gundabad remained an Orc stronghold, until it was cleared of orcs in TA 2799, after the War of the Dwarves and Orcs.
However, small groups of Mountain Orcs managed to return and fortified it anew, and menaced the Wilderland once again. It was from here that the Orcs and Goblins were present at the Battle of Seven Armies that came from in their marching numbers. Their leaders, Azog and Bolg were the supreme commanders of the Mountain Orcs from Mount Gundabad and presumably the northern Misty Mountains.
The Gundabad Orcs had appearance similar to that of the Orcs of Mordor, the footsoldiers of the Dark Lord Sauron. They had dark skin, fangs, claws, and a small amount of hair. They wore armor of brown and white fur coats threaded with various beast's teeth and Wooden shields bearing the Eye of Sauron. They, like many orcs, used long sharp swords called Scimitars with which they impaled and slashed their victims.