What is "gnipahellir"

In Norse mythology, Gnipahellir (Gnipa cave) is the name of a mythical cave. Gnipahellir is the location of Garmr, the hellhound who guards the gates of Hel, the Norse realm of the dead. Garmr is often featured chained here until the onset of Ragnarök, at which time his bindings break and he runs free. Reference to Gnipahellir appears in Vǫluspá, Prophecy of the Völva, one of the poems of the Poetic Edda.