Goblin Gate is in the High Pass over the Misty Mts. near Rivendell. There are at least 2 entrances - one (the back door) in the pass & another on the East side of the Mts.
For more information read the Hobbit - its where Thorin & Co. plus Gandalf & Bilbo are waylaid by Goblins (orcs) & kill the Great Goblin. Its also where Bilbo finds the Ring & has his riddle game with Gollum.