Gjallar is the Horn of Doomsday owned by Heimdall.

Magnus Chase and the Gods of Asgard

The Hammer of Thor

Magnus thought what would happen if someone accidently blew the horn. Heimdall told him that he used the horn for taking selfies and that it was his personal phablet. Heimdall had accidently headbutted the horn twice and all gods came running. He told Magnus that he informed the gods that it was a false alarm and made a .gif file of it.