Gjǫll ('loud noise') is the name of a mythical river in the underworld mentioned in the Grímnismál. It is the border to Hel, being situated nearest to the fence around the realm of the dead, which the bridge to the underworld, Gjallarbrú spans.
Gjǫll is also the name of the stone slab on which the gods fettered the Fenrisúlfr.
See also Gjöll