Gjallar is the Horn of Doomsday, owned by Heimdall.

Magnus Chase and the Gods of Asgard

The Hammer of Thor

Magnus thinks about what would happen if someone accidently blew the horn. Heimdall tells him that he uses the horn for taking selfies and that it is his personal phablet. Heimdall had accidently headbutted the horn twice and all gods came running. He tells Magnus that he informed the gods that it was a false alarm and make a .gif file of it.