The gargoyle gate isn't an actual monster, rather a full-metal sophisticated security gate found at the end of path B in Bilbaron Subterranean Fortress. The gate releases goblins and orcs as well as occasionally fire streams of flame. Be careful not to be nearby when the gate closes or you will take damage. The gate takes little damage, so you have to make use of the cannon to do massive damage. You have to fast, though, because the cannon will get targeted by the monsters, and if it's destroyed, you have to wait for another to come in.
Winning earns you the orange talisman.